As you purchased on Google Play, you need to simply connect your phone to your PC on the Videostream app (the app needs to be open on both devices so that your computer can 'recognize' your phone and the premium. Log in on your phone (which I assume is already signed in), then connect to your PC).
We did this so you can take Videostream premium everywhere (and because Google Play doesn't tell us who's purchased on their platform so we don't know who's a premium user (and therefore can't authenticate you or your email) if you purchased this way, because yay that's fun)! You need to connect your phone every 30 days for your computer to remember you have premium.
Hope this helps!