I just figured I'd post this tip for Windows Insiders running on Notification mode.

If you would like to activate your insider build and the Activation tab in the settings app isn't working, all you need to do is run CMD.exe as Admin and type the following command:

slmgr /ipk <product key>

Just replace <product key> with a working Window 10 Activation key and you should be able to successfully activate your insider build.

There are other commands as well and I will list them as soon as I get back to my home PC and boot up my VM to seek them out, but for now I'm currently at the local library so I don't have access to my VM.