When i enter the game it asks me for a player key?

[Charles "Tex" Watson]

I dont know whats this player key the game is asking me

[Craorach]

It's your CD key

[Minaro]

I'll also assume from the context that you might not know how to find said CD key;
Right click the game in your library list, and choose the option 'View CD Key'. There's the key you want.