Passing XML node value through a function argument

I'll be brief. I have an XML file with text data and I have a dynamic text field called "txt" located within a movie clip called "mc". I want to be able to load various text data from my XML file into that same text field. I made a function that creates an XML object with one argument. That argument simply passes along what nodeValue to load into the text field. Sounds simple enough right? When I run code below, it comes up as "undefined".

Code:

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function loadXml(theData) {
        var xmlObj = new XML();
                xmlObj.ignoreWhite = true;
                xmlObj.load("xml/about.xml");
                xmlObj.onLoad = function() {
                        mc.txt.autoSize = true;                 
                        mc.txt.text = eval(theData);  //loading the result of the argument into the text field.
                }
}

loadXml("xmlObj.firstChild.childNodes[0].firstChild.nodeValue"); // Loading function and passing the XML text location as an argument.

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However, down below when I don't use arguments and just put the XML location directly into function, it loads the text perfectly.

Code:

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function loadXml() {
        var xmlObj = new XML();
                xmlObj.ignoreWhite = true;
                xmlObj.load("xml/about.xml");
                xmlObj.onLoad = function() {
                        mc.txt.autoSize = true;                 
                        mc.txt.text = xmlObj.firstChild.childNodes[0].firstChild.nodeValue; //loading the same XML data directly into the text field
                }
}

loadXml();

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I can't figure out why I can't get it to work with the function argument which will give me more flexibility in what I want to load. Any ideas?