you can try this:
see if this helps:
summary:
it does a check to see if the cookie is set to TRUE (already been played).. if it is (FALSE) not true (or undefined...ie: not created) then it creates the cookie and sets the var to TRUE for he next 'check'.
Code:
//load your sharedObject called 'displayCookie'
var mySharedObject:SharedObject = SharedObject.getLocal("displayCookie");
if(mySharedObject.data.displayed == true){
//do whatever IF already been played
trace("cookie found");
}else{
trace("cookie not found, setting it now");
//do whatever if NOT already been played
mySharedObject.data.displayed = true;
mySharedObject.flush();
}