On install it should detect your CPU and GPU and set up slots for them both. It sounds like this didn't happen at all, so if you made the CPU slot you'll have to make the GPU slot too. You can do this in the Advanced client (the web is pretty useless I find, no options and it glitches out) under the Config -> Slots.
Note GPU is only used on Win not Mac.
Note not all GPU are made equal, nVidia will get more jobs than ATI, new will get more than old, etc.
Knowing your hardware specs would help advise.
Make sure you use a passkey! (Identity settings) Everything says this is optional, but you only get a tiny fraction of the points without one, so really it's required if you want credit.