Though it shares a subtitle, the Transformers: Dark of the Moon game is a prequel to the movie. According to High Moon Studios, the same team behind the excellent Transformers: War for Cybertron, the movie will not directly reference parts the game (as was the case with Disney's Tron: Legacy tie-in). However, if you play the game, certain plot aspects will be much clearer. For example, one of the levels on display explains how Megatron acquires his new semi-truck form from the next movie.
Following his defeat at the end of Revenge of the Fallen, Megatron found refuge with a group of Russians in Siberia. Many of the story details are still a closely-guarded secret, though the level showed a less-than-powerful Megatron working his way through the facility blasting away at an assaulting robot force. War for Cybertron's co-op gameplay is gone; the focus is instead on providing very directed experiences built around certain Transformers. Nearly every Transformer from the movie gets a chance to strut his stuff. For instance, Megatron's level focuses on his brute strength and ability to drain life from other robots.