Its free, and I
think it can import your drum samples. The packaged kits leave something to be desired, but if you're willing to set up your own, then it might be what you want.
Now the important question: does it run on Mac? Yes! There's a MacOS installer on the Hydrogen site.
http://www.hydrogen-music.org
Its definitely worth a look. Your can program individual measure-by-measure patterns, then arrange those sequentially into your song. Toggling between pattern and song modes is single-click. You'll also have a mixer strip for each drum , and the ability to (if you want) add plugins. There are also "humanize" and swing controls.