For those of you who are enrolling by computer:
After you finish and get the enrollment successful message, if you click the continue, you will be taken to a page that will give you an error message.

If you get ERROR: missing parameter PDP Year Missing MAPDY Year - this page must have a valid zip/count combo or a valid zip/county combo

Well, as the lady on the phone from Medicare told me: That's just something the computer does. Once you get the Enrollment successful, you did it.

In fact, I did it twice tring to fix the stupid error the page told me I had made.
I called Medicare to find out what was going on. She said they will just combine my two successes into one!

Good luck! (you might want to have a drink and / or a snack while waiting)
I really think it would be faster to enroll by phone!