The M2 Turret can be found in a few locations round the game, and are operated with the WiiU Gamepad.
The 1st is in the London Underground (the Safe House). It is located there to stop hordes of infected from tearing down the safehouse.
The 2nd is located outside Buckingham Palace, where it is used against a horde of zombies while trying to enter the palace. Why the horde appears is unclear.