Pop Up Button always has the same value in AppleScript
Have you checked out Stefan's answer to this question on MacScripter? 
He says to make a handler like this:

on didSelectItem:sender
    log sender's titleOfSelectedItem() as text
end didSelectItem


Then connect the handler to the selector on the popup button.

Link to MacScripter article: http://macscripter.net/viewtopic.php?id=43906