Minecraft mods for Mac?

My nephew is playing Minecraft on a MacBookPro running Mavericks and is trying to download the Pixelmon mod for Minecraft 1.7.4.

I found this thread about Minecraft mods, and understand that there are tools for adding mods, but don’t really understand how to get them loaded. Tried looking at a MagicLauncher thread on the Minecraft forums, but instructions for loading it on a Mac were…not really there (that I could tell).

Can someone either point us to a better thread or other resource?

Thanks a bunch!