Instructions
Select "Options" from the main menu. Choose the "Keyboard/Mouse" sub-menu. Enable the "Allow Developers Console" option, then go back to the main menu.
Start a new game on any level. Press the "~" key on your keyboard to bring up the developers console. Type in "Sv_Cheats 1" and then press "Enter" to turn cheat codes on.
Aim your crosshair using your mouse at the location you'd like to summon a Tank. Press the "~" key to bring up the developers console, then type in "z_spawn tank." This will summon a Tank to the location of your crosshair.