OK, go to
this link and click on the "Service Instructions". In that PDF file you'll see a picture like this:
(this is actually from the service instructions for the Nexus 8, but the picture is almost identical. Anyway, you are now looking at the service instructions for the Nexus 7, if you followed the steps above)
This should give you clues about how to adjust the shifting on your hub. Read the doc carefully, look at the pics and look carefully at your hub. It may be (I am sorry, don't remember anymore) that you'll have to remove the snapring and the sprocket in order to see the coloured dots align on the cassette joint. This is your homework now. But this is the part that needs adjusting in case of shifty shifting on the Nexus 7 and 8.
If you have to remove the snapring, be advised that it'll shoot off at very high speed. Take some precautions so that it doesn't gouge someone's eye out. It's a ***** to put back, too.
I made a tool taht makes removing and putting back of a snapring a snap (almost), but you don't have that tool so for you it will be hard. This may be a good reason to actually ask an LBS to help out.