First you need Combined Arms, of course, then you need to add a tank to the map editor. You will also need to add a "battlefield commander" slot, I think it's called. It's on the tool bar in the mission editor. I think it's the icon with the two flags facing away from each other. You can select blue or red side, or both.
Run the mission and select the tank. There is a button on the map somewhere that allows you to occupy the vehicle. I can't remember where it is.
This is all from memory, so it should be enough to point you in the right direction.