In MachFive 1.x:

If a loop you're playing back in MachFive doesn't match the tempo of your project, you can adjust the tempo by pitching the sample up or down. To do this, highlight the sample, then use the transpose and fine tune knobs in the pitch section of MachFive until the tempo of the sample matches playback of your sequence. A good method for this is to play back a click track from the sequnce while the sample is triggered via a MIDI track every bar or so ... adjust by ear during playback.

In MachFive 2.x:

You can still manually pitch samples, but a much better method is to use LoopLab's tempo controls. Set MachFive to sync to your host tempo, and any loops set to "tempo" or "pos" mode will automatically sync as well.