Beater is stuck on the mixer. No leverage when I

Spray some WD40 into the eject button. The wait 10 minutes and turn it upside down and spray WD40 down the side of the beaters Leave 10 minutes and try again/ If that doesn't work, you will need full disassembly

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Gently try to wiggle it out whilst pressing the button.

Try to wiggle it out with the button pressed.

The button doesn't move. You must push the beater up beyond th notch and turn it while pushing up.

Get a copy of the Repair Part List for your model at Kitchenaid.com
Constant pushing of the beater onto the Agitator shaft (40) can cause
it to be pushed in too far into the Planetary (33). With the beater in place,
use a rubber mallet to slightly tap the edges of the beater and push the
shaft out until you can see the retaining clip.
Now you have enough clearance on the shaft to push the beater upward
and twist it to remove it.

Poor some wesson oil into attachment socketin the head and work it a little by hand ----Unplug first. Work the release button and attachment.

I had this happen today. I took a towel and covered all of the beater except where it attaches. I sprayed WD-40 on it. This loosened it and it came unstuck. Then I washed it thoroughly with hot water and soap. It had some dried batter stuck inside which caused it to stick. Guess I better pay better attention to how clean I get it inside. PS Don't spray WD-40 anywhere else - this is something you make food with!

I had this same problem as well.  The beater was stuck like glue to the rotating arm, and it would not budge with any amount of pressure.  I tried the cooking spray and it did nothing.  But today I put a rag in hot/boiling water and then carefully wrapped it around the top of the beater.  I let it sit for 10 minutes. Then I put the rag in hot water again and repeated the process for about another 5 minutes.  When this was done, I covered the beater in a towel (to protect it) and forcefully tapped (ok... I pounded) it with the flat side of my meat tenderizer (it was all I had...you could use a mallet or hammer).  It took a little bit of work because I had to hammer it from the bottom, then the side, then the top to get the little spoke guided through the shaft.  But it did finally come off.  After cleaning with hot water, I put the beater on again and noticed that it was still a little hard to get on/off.  So, I put a little bit of veg. oil on it, and it seems to be better.  Going forward, I will be sure to clean the top of the beater & arm after each use and then coat it with a little oil to keep from sticking.  Best of luck!

I took the advice from: http://forum.kitchenaid.com/forums/topic.asp?TOPIC_ID=6356
I put a wash cloth in a pot of boiling water and used tongs to wrap it around the beater shaft while the mixer was resting on its side. When the wash cloth was not too hot to touch, I put it back in the boiling water and did it again. I was then able to easily remove the beater.

Try turning the beater in the reverse direction to the direction it normally turns - clockwise looking at the beater (I think), this should make the shaft pins ride up on some cams which should release the beater.