If for some reason the MacGameStore App has an issue such as it is stuck installing, downloads are queued but not downloading, or any other abnormalities, you can reset the app by following these instructions:
1) Hold down the OPTION KEY (alt on some keyboards) and keep it pressed down.
2) With the OPTION key held down, double click on the MacGameStore App icon to launch it.
3) A window will ask you if you want to reset the app. Click on reset.