The instructions are:
You put this in your podcast's rss xml code right below the <channel> tag:
<itunes:new-feed-url>
http://yourserver_here/your_new_feedhere</itunes:new-feed-url>
Now, you asked how to get at that .xml file. I don't use .mac, so this might not work. Look in your .mac folder for a file which ends with .xml. If you find it, download it and using a text editor like Tex-Edit, add the above code, save it, and put the file back.
The next time the iTMS updates, it will change to look for your feed in the new location.
If you cannot find it, then it's possible that .mac hides the .xml file somewhere, or perhaps it has .rss (?) at the end.
Hope this is helpful.