Instructions
Load the game created with Game Maker 6 in Game Maker 7. Because of the new formats, you will be able to load the game, but you will not be able to edit or play the created the game with the new software.
Click "Save as" from the "File" menu on the top toolbar. A standard Windows Explorer window will appear.
Click the "Save as file type" drop-down at the bottom of the window. A list of options will expand.
Click ".gmk" from the list of option and "OK" at the bottom-right corner of the window to save the file in a converted format for Game Maker 7. You will then be able to edit and play the game like you would any other game created with the seventh version of the software.