Look at the Help menu in the Editor - "Help on Using the Editor". This includes examples of adding both races and classes. It's also worth loading other creatures in the editor with New (Copy) to see how they were "made".
Races are pretty easy unless you have a heap of unusual racial/special abilities. Classes certainly take more work. If you have problems, start a thread in the forum and state what you have done, what's not working, and you'll get help.
