I don't think many people have worked out how exactly it works.... I am just doing my usual "Fred figures stuff out if the documentation is crap" (its an OCD of mine!)
I think probably the radio not working is not to do with logging in, i think thats just a system glitch. To fix MIB glitches, like in the Mk7, you can try holding down the MIB power on button for about 15-20 secs to force a reboot and that might fix it.
Plan B is that you stop, switch off the engine, lock the car for about 15-20 minutes and then try again - thats a real deep hard reboot. Its not ideal when you are trying to get somewhere, but it should get it back - it used to happen on the Mk7 too occassionally.
The login thing, definitely do that though or you will get "errors" - they aren't exactly errors but they will create warning messages and the "bong" after about 5 mins of driving EVERY time.
Login with your VW ID and make sure you've become primary user.