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to the right of the map there is a clear bar with three segments, you have to fill up a bar segment to use supports. how many full bars you have tells what skill you use. tier 1 skills need one bar, tier 2 needs 2, tier 3 needs 3. the best way i found to get the skills is to do team support actions as much as possible. you get little increase for kills, doing your battle cry seems to give you a bit more then kills.
then once you do have a full bar, just press the right button on the control pad to activate it.