Okay, bear with me here, maybe I'm just too stupid to correctly pinpoint the issue here but let me just explain my thought process on this.
I have two mods - Mo' Creatures and More Creeps and Weirdos. Mo' Creatures works, More Creeps and Weirdos doesn't. That one needs CustomMobSpawner to work, apparently, as the readme states. I start up a game, I see that More Creeps and Weirdos doesn't load, so I check the log and see that CMS didn't load because it's not a mod. Then when I check the minecraft forums, I see several other people are having the same problem as me, and apparently few people got it to work with and older version of Forge.
So with all this information that I've gathered, I thought the best place to go ask for help, is a Forge forum.