You might have Mod Loader installed, which if you do you have to delete it. You might also have other mods that are messing with Pixelmon. I don't really know a lot about mods since I just came I just came across this trying to find a tutorial for Pixelmon download. But tell me if this helps.
Make sure that you install forge first. With my experience, forge is a little iffy if you try to install mods before it. Also, I'm pretty sure that TMI and NEI don't work with pixelmon. That may be a different mod that I'm thinking of, but I know that I read that a popular mod didn't work with those two add-ons. Good luck!