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I have the keys in my library but I have no idea where to activate them. Are they essential to multiplayer?
This question / problem has been solved by dr.schliemannimage
To play Neverwinter Nights online, you need a unique CD key, while the game is installed with a generic one.
To change it, you have to input it in the Multiplayer Key Changer that you can find in the Start menu, or in Galaxy, under "MORE -> Other -> Multiplayer Key Changer/CDKey Changer".