Answer:
Gamehouse offer some good Mahjong games for Mac which you can try for free but you have to buy them if you want the full version. They are relatively cheap though. Check them out here http://www.gamehouse.com
For a free online version of Mahjong check out the Mahjonged game. It is by far the best online game you will find and it has loads of levels. Check it out here: http://www.mahjonged.com/mahjonged.html