While Dexter is waiting for Miguel Prado in his office, Dexter is playing a game on Miguel's computer. When Miguel arrives he takes Dexter's place and closes the program. However, when the camera angle reverts back to Dexter, the game is still visible on the monitor.
When Dexter is waiting in Miguel's office, he is seen playing a game on his PC, however the game was Halo 3, which was only released on the Xbox 360.
Dexter, while waiting for Miguel in his office is playing a first-person shooter on the computer called "Halo." FPS games played on a computer requires the mouse to shoot and rotate the movements of the characters you are playing with. Dexter has both of his hand on the keyboard as if he was typing.