Mods you need to install alongside with Mo' Creatures :http://www.minecraftforum.net/topic/612536-18-guiapi-an-advanced-gui-toolkit/page__pid__8422996#entry8422996http://www.minecraftforum.net/viewtopic.php?f=25&t=80246
1. Open minecraft.jar with the archive tool of your choice (make a backup)
2. Extract everything into a new folder
3. Add mod files into that folder
4. Delete META-INF
4. Do command+a to select all the files inside.
5. Right click and choose 'compress ... items'.
6. The resulting zip file will be called "Archive.zip" and will still be inside the folder.
7. Move 'Archive.zip' out of the folder to where the original minecraft.jar file was.
8. Rename it as 'minecraft.jar' (accept the prompt that asks you if you really want to do that)
9. Delete the folder you extracted.
That's what I got by doing some Googling, hopefully it helps.
The amount of archive compression and extraction tools on Mac seems really scarce. You could try Keka though :http://www.kekaosx.com/en/