FL for Mac
will run under Darwine flawlessly if you can get everthing going properly in darwine. If not. Apparently it runs well under parallels on a good machine with lots of ram.
The mac equivalent I'd say would be iDrum with Garageband. You make your beats with the idrum step sequencer, the same way you would in FL's SS. Save out all your loops (might be a more integrated way) then sequence in garageband. You then can beatslice your loops in the sequencer for more variation.