I am assuming you have a nowtv entertainment pass on your account and using a PC or Mac.
Then this is the method i use to reach the live entertainment TV section if using your internet browser on the NowTV website is to click the three horizontal small bars under Menu left hand corner of screen, then select Entertainment, then scroll to bottom of the screen where you should see the Watch live section with a row of live thumbnails.
Click on Sky Atlantic to open and play the live stream.
Alternatively once on the entertainment page, towards the top right there should be a small orange TV Guide box button, click on it and the Now & Next Live TV channels should appear on screen, then just scroll down to Sky Atlantic and click the live thumbnail picture.
If it was me i would test out the live Sky Atlantic stream ASAP before GOT starts at 2am just in case you encounter problems.