Once you collect the Progressive Bonus 4 times, you unlock one free spin on the Mega Wheel. Spin the wheel to automatically receive the Mega Bonus.
By the way - the prize you get is calculated based on two things:
The value of the prize that the wheel has landed on.
The amount of Facebook friends you have who also play LOTB.
So the more you play, and the more Facebook friends you have in-game - the bigger the prize will be!