When you start the VPN app for the first time, you should be shown a Start your Trial button. Below that should be a I already have a subscription link. Scroll down if you do not see it. Tap on that and your paid subscription should be recognized.
Are you saying Norton Mobile Security keeps stopping? What version of NMS do you have? Try going into Android Settings and look for any battery conservation settings. If you find that, turn off any optimizations for NMS and test.
[Admin Edit: Comment moved to its own thread]