First of all, you must have Windows 8 OS installed on your machine - this cannot be the consumers preview addition.
- Make sure that the section titled VS2012 x64 Cross Tool points to the "vcvarsall.bat" in your MS Visual Studio installation. Without this you won’t be able to successfully create Windows Store packages.
You should also note that for GameMaker:Studio to generate your app correctly, you will need a Developer Key. Please refer to the section Global Game Settings - Windows 8 Tab: Installation.
Finally you should click on the Check Windows 8 SDK button to confirm that your selections are okay and that everything works as it should. If you get an error or you are not sure that your Developer Licence is correct, you may also click on the button labelled Acquire Developer Licence which will open up a command prompt and automatically go through the necessary steps to get you one.