Hey man, You should be able to convert AIFF into MP3 with iTunes.
Try this...
Go to preferences and click advanced.
Then from the 'Import using' drop down menu, choose MP3 Encoder. Then press OK.
Now select the track you want to convert and go to advanced then convert selection to MP3.
iTunes should create a new track with the same name and everything but this would be an MP3 file. Drag it to desktop and there you will have your file.
Hope this helps!