If you were playing the game log off Warframe, and also log off your Steam account. Log in to Steam with the credentials provided in an email you received after purchase or in your library on our store. Log into your Warframe account (using your own login and password) from the Steam account of the seller. You will see information: You have pending Steam packs. Would you like to aply them to this acount? Click: AcceptIf you're requested to Link to steam economy - Decline Once you accept the Steam pack the content will be added to your Warframe account. After this relog to your Steam account and double check the added content. DLC is not visible in Steam library, it will be visible in-game in Warframe.