Here's the link:
http://mytreo.net/treofaq/RingtonesHelp#7
It's actually for older version of the Treo, I think, but the 750 does work with MP3's. I'd like to know if an MP3 conversion to WAV would make a smaller file size. I'd do the conversion of that was the case. With the limited memory o these things anything that saves space is good.
I emailed the MP3 to myself from my desktop and picked it up via email on the treo. I didn't try to just drag it over on the PC using activesync - or you can use bluetooth or IR.
It says to save the MP3 file onto your SD card for space saving reasons, but I saved it directly into the Windows folder. Any folder on the treo internal memory would do though. If you save the MP3 on an SD card and you have multiple cards that you swap in/out, then if you don't have the right card in the unit, your ringtone won't work.