If you have time, take off the nut and spray a little PB Blaster or Kroil on the exposed shaft area a couple of times over a day or two. Put the nut back on (not tight) and (not too hard) hit the arm from the side to help loosen before using the puller.
To add to Bill's procedure, with the nut flush with the end of the shaft, install the puller and put some tension on it. Then hold a heavy hammer (4 lb sledge would be good) against one side of the arm, and smack the other side of the arm with a 1 lb hammer. What that does is make the rust molecules that are holding hands in there let go and cover their ears.
Usually make things come right off. That works very well for tapered stem items like ball joints or tie-rod ends.
