The button used to deploy a parachute varies depending on the version of the game. For the PC, press the left mouse button. For the Playstation 3, press L1, and on the Xbox 360, press the left bumper.
These controls are the game's default button configurations. Players can change the buttons used through the game's menu.
The button used for opening the parachute is the same button used to fire weapons. For the parachute to open, the player must also be falling and have the parachute equipped as an active item. The parachute will auto-equip if the player is falling and does not have a weapon equipped.