I only know how to work this for PC, sorry.
Make sure that under folder options who have the "Hide extensions for known file types" UNchecked.
Then, make sure whatever song you want to use as a ringtone is in AAC format, if it isn't, you can simply click on the song in iTunes and change the format..easy.
Then go to "add file to library.." and find the AAC file you want as a ringtone...at the end of the file will be the extension ".m4a"; simply change it to ".m4r" and upload. You won't see it in "Music", but if you look under the "Ringtones" tab on your iPhone you will see the file is now on the list to sync to your iPhone.

I have done two files so far, it works great!