There's a handy update button built right into the Wii's system settings. Plus your Wii will even prompt you to install updates from a disc that needs them. Amazing how Nintendo were considerate enough to include those features, and not rely on people to manually download updates on a computer.
If you must do it manually, I believe there were some cross platform Python based tools. NUS WAD Packer or something like that was one, written by Xuzz using wii.py. Might have been other stuff too, not completely sure.
http://gbatemp.net/t...r-v10-released/ - Mac link is broken, source link is there. If you install Python and any other needed stuff, source should work. The compiled Mac version might have made it easier, but it should be possible to run it from the source.
EDIT: Instructions for use on Mac: http://gbatemp.net/topic/174577-nus-wad-packer-v10-released/page__view__findpost__p__2195239