Using power ups requires the meter on the right side of the screen to fill up during matches. You must also have the green balls with the ! in the center of them in order to activate the power up.
The more matches you make and the more time that passes will give you more chances to fill up the bar.
I suggest playing on the higher level areas or at off times of the day. The fewer people in the public lobbies means the matches will last longer and you will be able to activate more power ups.