I make custom ringtones on my iPhone using this helpful guide (and a Windows PC). Maybe it'll help you? Try this:
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Make an iPhone ringtone in iTunes 10
1) Launch iTunes 10
2) Find and select the song you want to make a ringtone out of in iTunes 10
3) Right-click on the song name and select Get Info, then click on the Options tab
4) Select the playback period of the song that you want to ringtone to be, make sure its 30 seconds (or less)
5) Now click OK and then right click on the song again, and select Create AAC version to create a new version of the song with the 30 second interval you specified
6) Locate this newly created 30 second clip in iTunes (at the top of the playlist if you search by date added) and right click on the file and then select Show in Windows Explorer
7) Now in Windows explorer, rename the file extension from .m4a to .m4r
8) Accept the file extension change to .m4r
9) Now back in iTunes remove the file from the playlist (do NOT move to Trash, select keep file) and then re-import the file into iTunes 10 by double clicking the .m4r file within the Finder or Windows
10) The file will now be added back into iTunes as a ringtone and you can do what you want with it
11) IMPORTANT! - Dont forget to change the start/end time back to normal [on the original song] after you're done, or every time you want to listen to that song it will only play 30 seconds!
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Of course, you'll have to load that MP3 into iTunes, but then it should work. Let us know how you get on.
Marilyn