This is their registration page.
Or go to their home page at HUD.gov for more info. On the very right hand side of the page you will see a section labled: Information For.... , go down to the Housing Industry section and under there you will see in mice type appraisers. Click on the "appraisers" section to look up resources and ax questions.
P.S. Once you are approved, you have got to remember to fax a copy of your renewed license into the REAC every year. They wont ask for it or send you a notice, you just have to remember. If you forget, it is a pain to become re-activated and almost takes an act of Congress. Lesson from experience.