Well.....your Mac should come with Sound Studio. Try that.
EDIT: If all you want to do is crop out parts of a song, use iTunes. Just highlight the song, Control-click->Get Info. Click the Options tab, and check the boxes marked Start Time and Stop Time. Type in the times that you want the song to start and stop. Then, to save a permanent version of the crop, just re-import the song by converting it to whatever format you like.
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