first of all, its the heart next to the game title and second, this should be locked because if anyone else adds something after i told you how to do it, its considered spam, im told.
The "fave" button is a little heart that it straight across from the title of the game. It's easiest to find while the game is loading and there's nothing on the screen to distract you. It's there with several other buttons. If you still can't find it, please let me know.