You should be able to left/right click on the race icon (should say random by default) and it will switch. There might be a slight delay, but it'll do it. You can do the same by clicking on the AI difficulty and a spot on the left which will determine the type of AI and let you close or open slots.
If that doesn't work, sins doesn't like ur comp, or your mouse doesn't like sins. I'd reinstall it to see if it would help.
