I have a GE '93 model dishwasher that doesn't empty all of the rinse
water.  It leaves about 1/2 to 3/4" of water in the bottom of unit. 
I'm not very mechanical so if this is something I should'nt try to
fix, is there a better way of explaining this problem to a plumber and
is it very expensive to have repaired or should I invest in a new
dishwasher!?

Read this:

http://www.applianceblog.com/forums/forum6/892.html

I would not invest in reparing a 13 year old dish washer.  It has
likely passed it's best years.  Improvements to dishwashers over the
past decade and the relative cost of buying a new one vs paying for
plumbing repairs makes the decision to replace it pretty clear cut.

Disconnect the dishwasher's drain hose from the sink's trap and lower
the end toward a basin.  Then clear or cancel button on dishwasher to
drain the machine.  While draining, shake and bounce the drain hose to
get out the clogged food (probably!).  Good Luck