I've seen this.
There may be an easier way to do it, but I have to do the following:
1. From the www.adtran.com webpage, click on "My Adtran", then sign in.
2. Click on "My Products".
3. Scroll down to your registered device, and click on the hyperlinked part number. This will take you to a specific product page for that product with Enhanced Feature Pack.
4. Click on the "Support Tab"
5. Click on the link under "Software Downloads"
6. Continue through the normal process of downloading firmware.
I hope this helps.
Alternatively, email Adtran support and explain you do NOT require support from an engineer - only someone to send you a link to download the firmware in question, or unlock your access to it on the website and you can deal with flashing the unit with the firmware. However, I suspect it may be a browser interaction with the download page which is causing the problem, since you already have registered your product.
I had been using Chrome. I tried IE. Same issue. I am going to try and contact them again unless anyone else has suggestions on this. Very frustrating.