If you find that you can’t edit the “Get Info” fields of an MP3 file in iTunes try this:
Right click (Ctrl-Click) on the file in iTunes and then click on Create MP3 Version. This creates a copy that is editable in “Get Info.” You can consider trashing the original file or keep it just to be on the safe side.
To find the original file, right click it in iTunes, and then click on “Show in Finder.” Then do what you want with it.
This all worked for me on two files I downloaded for my podcast from Mevio’s Music Alley.
The JogTunes Indie Podcast