Once you have subscribed & downloaded a game, the game can be accessed by following these steps:
Select your game from the Steam WORKSHOP. Click 'Subscribe'. - You MUST be subscribed to games you wish to play.
Open GameMaker: Studio in Steam. You will be given the option to Play Or Create - Select Play.
You will be given a list of games you are subscribed to. Select the game you wish to play.
Select 'DOWNLOAD' in the bottom right hand corner.
If you already have Game Maker: Studio (creator) running:
Open GameMaker: Studio & Select the 'Play!' button on the top right hand side of the tool bar.
This will open a list of game you are subscribed to. Select the game you wish to play.
Select 'DOWNLOAD' in the bottom right hand corner.