I found a solution to my problem, so I thought I'd post it for others to find...

The problem was that Garmin never sent an account activation for my original registration, nor for the follow-up account I created to test the process. Here's what finally worked: I used the option to resend the account activation code (search on Garmin's support page). The instructions say you can simply enter the account e-mail and request a new code, but I had to fill out all the fields, including username, so I essentially had to create a new account on the same e-mail.

Nick