I'm having problems too; this used to work!
Right now, I try to activate via hbogo.com/activate but then it takes me to a page asking me to buy HBO On Demand (http://xfinity.comcast.net/instantly/watch/?cid=hbo&p=d) even though I already pay for HBO Go!!!
This is really ridiculous. I'm not able to activate HBO Go for the AppleTV even though I'm paying Comcast money for it.
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