I think I solved the problem!
Not sure how, but I will tell/show what I've changed;

First of all make sure you load the swfaddress javascript file after the swfobject file file in your index.html like this:
PHP Code:
<script src="js/swfobject.js" type="text/javascript" charset="utf-8"></script>
<script src="js/swfaddress.js" type="text/javascript" charset="utf-8"></script>

<script type="text/javascript">
    var attributes = {
        id: "myid"
    };
    var params = {
        scale: "noscale",
        wmode: "transparant",
        allowscriptaccess: "*"
    };
    swfobject.embedSWF("myswf.swf", "flashcontent", "100%", "100%", "9.0.0", "js/expressInstall.swf", attributes, params);
</script> 
Not sure if that was the problem, but it's recommended.
Secondly, I forgot to upload the SWFAddressEvent.as file, I only uploaded the SWFAddress.as file.

I also adjusted my AS a bit:
Instead of calling the activatePage function directly on the menu buttons, I set the SWFAddress Value and let the SWFAddress.onChange function call the activatePage function.

My new AS:
Actionscript Code:
// SWFAddress include
#include "com/swfaddress/SWFAddress.as"

// Load XML
var myXML = new XML();
myXML.ignoreWhite = true;
myXML.onLoad = function(success) {
    if (success) {
        makehoofdmenu();
        SWFAddress.onChange();
    } else {
        trace("The XML could not be loaded");
    }
};
// Prevent XML caching
myXML.load("content.xml?cachebuster=" + new Date().getTime());

// Menu
var tmpWidth = 25;
var spacing = 20;

var hoofdmenuitems:Array;
var hoofdmenutotal;
var page:Number = NaN;
var pages:Array;
var tmpl:Array;
var button:Array;
var activeNo:Number = NaN;

function makehoofdmenu() {  
    hoofdmenuitems = myXML.firstChild.childNodes[1].childNodes;
    hoofdmenutotal = hoofdmenuitems.length;
    pages = [];
    tmpl = [];
    button = [];
    
    for (s=0; s<hoofdmenutotal; s++) {
        
        pages[s] = hoofdmenuitems[s].attributes.title;
        tmpl[s] = hoofdmenuitems[s].attributes.template;
        button[s] = hoofdmenu.items.createEmptyMovieClip(s, hoofdmenu.items.getNextHighestDepth());
        button[s].createTextField("title", 1, 0, 0, 1, 15);
        button[s].title.setNewTextFormat(Avenir);
        button[s].title.embedFonts = true;
        button[s].title.autoSize = true;
        button[s].title.html = true;
        button[s].title.antiAliasType = "advanced";
        button[s].title.selectable = false;
        button[s].title.htmlText = pages[s];
        
        // Aligning
        button[s]._x = (button[s-1]._x + button[s-1].title.textWidth) + spacing;
        
        // Functions
        button[s].buttonNo = s;
        button[s].onRollOver = function() {
            if(this.buttonNo != page){
                select = new Color(this); 
                select.setRGB(0x000000);
            }
            activeNo = this.buttonNo;
            SWFAddress.setStatus(pages[activeNo].toLowerCase());
        }
        button[s].onRollOut = function() {
            if(this.buttonNo != page){
                deSelect = new Color(this); 
                deSelect.setRGB(0xFFFFFF);
            }
            SWFAddress.resetStatus();
        }
        button[s].Status = 0;
        button[s].onRelease = function() {
            this.Status ++;
            activeNo = this.buttonNo;
            if (this.Status > 1) {
                this.Status = 0;
                activatePage(activeNo);
            } else {
                SWFAddress.setValue(pages[activeNo].toLowerCase());
            }
        }
    }
};
    

// SWFAddress handling
SWFAddress.onChange = function():Void {
    
    var value = SWFAddress.getValue();
    // If URL is direct check witch page it is
    pagename = value.substr(1, value.length).toUpperCase();
    getPageURL();
    
    // HTML Title
    var title = 'MY TITLE'; 
    var names = SWFAddress.getPathNames();
    for (var i = 0; i < names.length; i++) {
        title = 'TITLE - ' + names[i].substr(0,1).toUpperCase() + names[i].substr(1);
    }
    SWFAddress.setTitle(title);
}

// Activate Page from URL name
function getPageURL() {
    for (s=0; s<hoofdmenutotal; s++) {
        // Loop through pages to check witch is the right one
        pages[s] = hoofdmenuitems[s].attributes.title;
        if (pages[s] == pagename) {
            activeNo = s;
            activatePage(s);
        }
    }
}


// Activate Page
function activatePage(p:Number) {   
    var showFunctie = eval("show"+tmpl[p]);
    var hideFunctie = eval("hide"+tmpl[p]);
    
    // If it's a link
    if (tmpl[p] == "link") {
        var link = hoofdmenuitems[p].firstChild.nodeValue;
        getURL(link, "_blank");
    // If it's a page
    } else {
        if (isNaN(page)) {
            showFunctie(p);
            selectActive(p);
            page = p;
        } else {
            if (page != p) {
                if (tmpl[page] == "werk") {
                    randomBG();
                    var movie:MovieClip = eval("hoofdmenu.content."+tmpl[page]);
                    movie.removeMovieClip();
                    eval("imageflow_"+current).removeMovieClip();
                    image_info.removeMovieClip();
                } else {
                    var movie:MovieClip = eval("hoofdmenu.content."+tmpl[page]);
                    movie.removeMovieClip();
                }
                showFunctie(p);
                selectActive(p);
                page = p;
            } else {
                deselect();
                hideFunctie();
                page = NaN;
                activeNo = NaN;
                SWFAddress.setValue("");
            }
        }
    }
};

I hope I can help someone with this piece of AS!
Cheers