So what can't you do with SndSampler/QuickTime? The main thing is that you can't (at the time of this writing, at least) take a non-MP3 audio file and convert it into an MP3. But you can play already existing MP3 files, and convert MP3s into other kinds of audio files. Once you convert an MP3 to a format that SndSampler can understand, you can edit the MP3. (Although, strictly speaking, it's no longer an MP3.)
Now, assuming you've got the right version of QuickTime, in order to play an MP3 you go to the Sound menu and choose "Play File From Disk." When the standard file dialog comes up, choose your MP3 and click OK. After that, QuickTime will do its best to play your MP3. As you might have guessed, sometimes QuickTime will encounter an MP3 which it can't play. In such a case, the only thing you can try is upgrading to a newer version of QuickTime, if such is available.
What about making MP3 playlists? Well, yes, this is something you can also do with SndSampler, but you have to do some work in the Finder first. The best thing to do is store all your MP3s in one central location on your hard drive. Playlists can then be built using folders and aliases. To build a playlist, first go to the Finder, make a new folder, and name it as a playlist. Now, select all the MP3s you'd like to have in that playlist. Make aliases of them, and drag these aliases to your playlist folder.
Now, when you want to play those MP3s, just drag and drop the playlist folder onto SndSampler's icon while holding down the shift key. The MP3s will play automatically, one after the other. They'll even play in the background!
Now, go to the File menu and choose "Import." Use the standard file dialog to choose your movie. Make sure the "raw audio" and "ASCII" checkboxes are cleared, then click OK. You should then see the standard QuickTime file conversion dialog. Click on the "Options" button to change the format of the audio data, then click OK when you're done. The MP3 should be converted to an AIFF, which you can then open and edit with SndSampler (assuming you have selected an appropriate audio compression scheme--SndSampler understands IMA, A-Law, µ-Law, MACE 3:1, MACE 6:1, and of course "none," i.e. uncompressed).
As we mentioned before, sometimes QuickTime will encounter an MP3 which it can't deal with. There's not much you can do about it, except maybe upgrade to a newer version of QuickTime. Remember the golden rule of Macintosh programming: if it doesn't work, it's Apple's fault!
Now, unfortunately QuickTime (at the time of this writing) doesn't allow you to create MP3s. So, once you've edited your (now no longer an) MP3, you can't convert it back into an MP3 with SndSampler. You'll need to get yourself an MP3 encoder.