If the pin won't budge -- you can't tap on it and get it to move -- then that's your problem. Do you think it got jammed up when you were "tapping" on the extender to put it on the shaft? Was it depressed (pushed in) when you started this extender on the shaft and DID YOU SEE IT DROP INTO THE GROOVE and extend when you got it "tapped" into this position?
Shouldn't take a lot more force to remove it than it did to tap it on . . . but if that's locked into the detent on the tractor's pto shaft, you'll have to start there.
I've never taken one apart, but it looks like removing the C-clip on the pin would be the first place to start. Then I'd pull on the top of the pin with some channel locks or vice-grips or get a screwdriver under the head of it and tap lightly. If it won't move in that direction, try tapping it in the other direction. Spray liberaly with penetrating oil along the way. Once it starts moving, use a punch to drive it out. If you hit resistance that's enough to worry you, turn it over and try driving it the other direction. There must be a pretty simple way to get that spring-loaded pin out of the housing. It must be jammed against the splines but hard to say which direction you need to go.
Unless the pin is "locked" and holding it on, I still say that tension on a ratchet strap and then tapping on the sides of the extender should get it to release.