Shannon,
Prometric allows you to schedule (for a small fee... I think maybe $25) a "mock test" where you go the the center, go thru the process of check-in, etc. and take a mock exam (not related to the PMP.
I did this a few days before I actually sat for the exam, and for me it reduced my anxiety level considerably (I don't have ADD, but I was much more comfortable knowing where to go, what to expect, etc.).
I was still nervous, but considerably less so than I would have been! Thankfully, I passed (which I largely attribute to the exam simulator).
Best of luck to you.
Tracy