I am not with Laminar Research; just a flight simmer.
Firstly X-Plane is not a game. It is a serious simulation package.
Steam is 3rd party hybrid of the official Laminar Research (LR) version of X-Plane taking a commercial advantage of the development work carried out by LR. There is no connection between either party.
Although I don't support steam and normally do not respond to steam problems I understand you do not have to specifically add any additional packages as the hybrid contains all of the LR files.