I think you have to install the receiver, activate and then you can pull it from your system. I had to activate mine in order to access some other streaming content from comcast.

Update:

I figured the best thing to do to solve the problem was wait for my cable box to arrive in the mail, activate it, and then see if I could sign in and access HBO Go.

In the meantime, this whole thing started because I upgraded my internet package. I did some speed tests and realized I was getting the same speeds I was before the upgrade. You can probably guess where this is going. Yep, I called tech support (it pains me to write "support" there) and spent another two hours getting absolutely nowhere.

I couldn't take it anymore. I finally went ballistic and cancelled the whole thing. No HBO Go, no super fast internet for me. Do I still have internet with Comcast? Of course I do, because they're the only game in town. It's not a monopoly though.....! ARRGGHHHHHHH!