I cannot unlock the mixer to release the mixture in the bowl, suggestions?

Kitchen Aid suggests putting a dime in the bowl and turning the mixer on low. If the beater attachment pushes the dime around the bowl, the attachment is too low. If dime doesn't move, the attachment height is too high. When the attachment height is just right, the dime will only move a little.

why are you over loading the machine against the user manual advice
contact hobart for spares or advice on the problem
cut back on the quantity of ingredients in use and make the mixture to the right consistency according to the recipe
if you still have a problem .take it to a hobart authorized repair shop and have it fixed

The only way to unstick it is with more energy. Get someone to hold the motor unit firmly and someone else turn the bowl anti clockwise. There is no magic if it's stuck, just more effort required.

Try making up a thin disk or two to put under the bowl to lift it up. Over time the head locking mechanism can wear allowing the head to lift up a bit.

If that is the bowl that came with the machine it is not wrong. What happens from time to time is that the head on the mixer automatically adjusts causing the beater to not operate as it should. To adjust the head on the mixer place a dime at the bottom of the bowl, attatch the beater to the mixer and make as if you are going to start mixing. The beater should JUST touch the dime. If it doesn't touch it at all or is pressing it against the bowl, tilt the head of the mixer up to locate the adjustment screw located inside the shaft. Take a flat head screw driver, turn the screw clockwise (to lower the head) or counter clockwise (to raise the head). Place the beater back on the mixer, turn the machine on and see if it moves the dime moves. Once it is properly adjusted the dime should move ever so slightly.

There's an adjustment screw as shown in the picture.
http://www.fixya.com/uploads/images/07F1BE1.jpg
Screwing this screw clockwise will raise up the bowl so the mixer paddle reaches closer to the bottom. Don't go too far, the paddle will start hitting the bowl.

Try running the mixer for a few seconds to clear anything out from under the beaters, like a chocolate chip. You can also try pushing down on the beater end of the mixer, press the button and lift.

you may have the old style frame which is why the bowl jammed old bowls are no longer avaible you may need to purchace a new mixer

Go to sunbeam.com, select "kitchen", then select "customer service", then "instruction manual", then "stand mixers", scroll down to model #

You have to twist the bowl anti-clockwise to release it from the base. Twist clockwise to lock it when using the beaters.