Machine Gun: To use it, you have to be an engineer (last unit from list). Then, stand close to the MG and press E. Shoot using Q, to aim, use WASD. To leave MG, press E again.
Flak turret:
Exactly the same as the MG.
Tank: Look at the tank's edge, find a ,,hp bar''. Hold F to get in. First person that gets into the tank is a driver, the second is shooter. Do not use tanks alone! To drive, WASD. To shoot, E (Machine Gun) and Q (cannon). To switch your position (driver - shooter), P. To leave the tank, B.
Truck: Look at the door, hold F as the hp bar arrive. You won't be able to enter the truck unless no one is standing on it's back! To drive, WASD, to leave B.