Hi Anni,
No wonder! You named your frame "mainFrame" in the frameset, ...
<frameset rows="119,*,32" onLoad="fillFrame();" cols="*" frameborder="NO" border="0" framespacing="0">
<frame src="top.html" name="topFrame" scrolling="NO" noresize >
<frameset cols="153,*" frameborder="NO" border="0" framespacing="0">
<frame src="nav.html" name="leftFrame" scrolling="YES" noresize>
<frame src="main.html" src="javascript:parent.blank" name="mainFrame">
______________________________________________________________^^^^^^^^^^
</frameset>
<frame src="bottom.html">
... but on the other hand you call it "main" in the function.
function fillFrame() {
parent.main.location.href = pageURL;
//^^^^
}
Simply give it the same name in both cases and your script will work without errors.
Gruß Gernot