Hello I bought napoleonic wars DLC in GOG galaxy, but I cant play in multiplayer because it keeps asking me for a SK but I dont have any serial key please someone. Help!

Galaxy is not feature complete. You need to login from a web browser to your account, access your library and in the game card hover over 'More' and there should be 'Serial Keys' as an option.

Yeah, but when I download a mod, I can't play it because, it puts Your Napoleonic wars DLC cant be found