As far as I remember, you first apply on the ADA website:
https://www.ada.org/prof/ed/testing/dat/online/index.asp
Once you do this part read this:
http://www.ada.org/prof/ed/testing/dat/registering.asp
ADA will send you the 800-number and your confirmation # and then you call up the prometric center near you and schedule a test date.
here is the prometric website so you can see what is near you:
http://www.prometric.com/Default.htm
if you have any more questions or need me to hold your hand PM me.
🙂