Pretty sure you have to go to your inventory ( Spacebar by default) and click on the spray can first to equip it. You might have to find the cans first in the maze, in order for it to work.
There has ben some problems regarding in using it, and best way to fix it, is to buy the spray can from the arcade mode first, in order to make it work. But once you have it, its just using your right click butten to shoot the spray.