I'm using GUIMiner. It's not using my CPU. My CPU is at a very low utilization (~10%).
What I can do to use my CPU?
Why isn't CPU mining turned on by default?
CPU mining ended in 2011. You will earn less than what the electricity costs, so the more you mine the more money you lose. You will never earn the minimum amount required to cash out from a pool, so you actually won't be earning anything at all, just burning electricity.
This is why CPU mining is disabled, and why noone should use it.