You can use either the directional pad or the left analog stick to perform combos. If you are using the directional pad, all you need to do is enter the combination. Nothing more. But if you use the left analog stick, you need to enter the directions then click down.

Be Invisible	left, left, down, down
Energy Shield	right, right, down, down
Normal mine	right, left, down
Charged up Mine	right, left, down (hold down for two seconds)
Freeze	up, down, up
Freeze Attack	right, left, up
Shoot weapons from the back on the car	left, right, down and button to fire weapon