Problems with Soundtrack Pro

In class we have been using Apples Soundtrack Pro to do audio work. On a recent editing exercise we had problems exporting from Soundtrack as an .mp3 file. Naturally the solution is simple but not, I think, obvious. When exporting from a multi track project there's no problem. If you try to export from the Wave Editor you can't export to .mp3. This was what we were trying to do. Instead you have to import it to an empty multi track project THEN the export option allows you to export as .mp3 as long as you have selected master mix! Phew!