For anyone purchasing the internet only package which includes HBO Go, you must have a DCT box connected even if you don't intend to use comcast cable on your TV!
Much like everyone in this post, initially (with only a cable modem connected) I was able to sign into my comcast email account then use that information to log into the HBO Go website and successfully watch any show.
Few days later I received the good old 'ERR-36895' error message and couldn't sign into HBO Go. Comcast support hadn't a clue why and after about 3 weeks of failed attempts I was called by Tier 2 tech support who correctly informed me that I need a DCT box hooked up to register my account with HBO servers as requiring access.
A DCT box is a non-HD box that lets you hook up Comcast to your TV and allows you to access advanced features like on demand and above basic channels like HBO (a DTA box is for absolute basic cable only) .
I'd strongly suggest requesting a Motorola DCT700 box since it's small, non-HD and no additional cost or monthly rental for us. They initially sent me a DTC2000 box which is an ugly mamoth larger than a VHS player. Unfortunately Comcast couldn't guarante a DCT700 would be shipped to me so I had to visit a local service center to receive the correct equipment.
Find it on this page and see that it's the only small non-HD box.
http://customer.comcast.com/help-and-support/cable-tv/cable-box-user-manuals/
Once you receive it, if you only have one cable exiting your wall then you'll need a splitter (I had one from my previous house but you might be able to request one from Comcast when you request the DCT box) to split the connection into both your cable modem and your DCT box. I didn't need to connect the box to my TV (just the coaxial cable and power) and then I called Comcast to ensure the box was activated on my account by giving them its serial number. Then specifically ask them to "send a signal to reset my DCT box" and if your DCT box resets you'll be able to immediately log into HBO Go successfully.
This internet only HBO Go package is brand new so I guess early adopters like us need to perservere. I created this account to specifically post this information as it was not available any where else.
Good luck folks 