Why won't Minecraft mods work?

I have a mac and Minecraft 1.6.6 downloaded. I've read over all the guides on what to do but a lot of mods don't work. I managed to get Single Player Commands and TooManyItems to download, but when trying stuff like Humans+ and Millenaire(sp?), it simply doesn't work. I have removed the meta_inf file or whatever it's called. If any has a solution, I would be very grateful!