Easy enough to unlock :)
I unlocked this on Nokia 520 by tapping the bottom of the screen in the main menu where you can scroll through puzzles, medals, achievements etc.
Tapping the bottom of the screen will bring up some extra options at the bottom of the screen including Settings. Select Settings and then select the option "Show available tiles" to ON.
return to puzzle select and complete a puzzle.
Please feel free to comment and any neg votes then please let me know where I need to improve this :)