If you have Nero CD burning ROM software, it comes with a wave editor.

All you have to do is open a file in that program and then save it again as a smaller mp3

So when it is open you click "save as" and then you are given the option of saving it as a smaller mp3, which you accept according to specifications you have chosen.

It may be possible to follow a similar type of process on your mac.

Do you have a guitar icon on the bottom of your screen?

If that is (as I believe it might be) a music software program, it may be possible to open an mp3 file in there and then alter it before saving it again, or indeed (as above) to click "save as" and save it as a smaller file (I hope I have explained the difference between these two processes helpfully).

Sadly, not being a mac man I cannot do more than give an educated guess.

Good luck.