On top of the air piston. Easiest way is to let the air out, unscrew the air valve and pour it in through the top. Start with small amounts, maybe 2-3ml. I put in about 6ml and it was too progressive for me. Had to take most of it out. Re-set your PSI to your desired sag, it will probably be a few PSI less.
Another mod I enjoyed is to carefully pry up the dust wipers and put a bunch of marine grease or slick honey under them. The 55CR's are sealed to the max, so not a lot of oil gets up onto the stanchions and they can get a little sticky on small bumps.
Hope this helps, I am really loving this fork.