Can't activate my Windows Tablet

Hi!
I just bought this tablet (ativ 500T) ,the version with GPS and 3G.
The guy who used the tablet installed (i don't really know how) windows 8.1 Pro N( The tablet comes with 8 PRO)
I got the OEM key but I can't activate the windows,it says something like the key can't be used on this windows version..
What can I do?

http://www.eightforums.com/tutorials/18309-windows-8-windows-8-1-iso-download-create.html

Pro N is a different version, and won't activate with a standard Pro Key.

The Pro installation should install seamlessly and activate as soon as you go online.

Superfly said:

http://www.eightforums.com/tutorials/18309-windows-8-windows-8-1-iso-download-create.html

Pro N is a different version, and won't activate with a standard Pro Key.

The Pro installation should install seamlessly and activate as soon as you go online.

Click to expand...

Which one do I have to install? The 8.1 or 8?