Setting up Game Creator is really easy.
Open the Asset Store and search for Game Creator
Download the latest version and check your Unity version is compatible
Open your Unity project or create a new one
Click on Open in Unity and it will automatically install
Once you have Game Creator imported in your project you can start developing your game.
If this is the first time you're using it, we recommend you check out the Example Scenes. To open them, simply open the Module Manager [⌘ + Shift + M] and click the Enable button in the option Game Creator Examples.