If you have enough points, you click the image of the unit you want to call in, and your cursor will change to show you that you are now placing the reinforcements. It looks sort of like the arrow is now pointing down to the ground...That's the best i can describe it
Then you just click anywhere on the ground where you want them to move to. The time limit tells you how long until you can take control of the unit.
Remember, they always come from the same direction, usually behind you (so they don't get shot to pieces by the enemy before you get to use them). So keep that in mind and don't send them too far out into the battle, it's best to bring them up into your lines and then you take over control.
If you can't call in the specific reinforcement it will be grayed out. This usually means either you haven't got enough points, or the game hasn't progressed long enough for the units to become available. You will notice the second more in Multiplayer though, to stop people from bringing in the big guns/tanks at the start.