Does BitMinter not use CPU's for mining at all? I just started using BitMinter on several computers. They have decent GPU's that are detected and working fine, but I'd like to be able to use my CPU's also and I can't seem to find any method to enable it.
GPUs are obsolete for mining. You won't get any reward worth even close to your electricity and hardware degredation cost. This has been true for over a year at least.

CPUs are 1000x more wasteful.

You're too late to the home-mining game.