I  can't turn the key lock down on the left side to get under the seat.  I have sprayed it with penetrant to no avail.  Any suggestions for how to loosen the lock cylinder?

Thanks!  Rick

Have you tried pushing down on the seat to release some of the force on the latch?

Same as Triple Jim, push down on the seat so that there's less tension on the latch. After that you may have to remove the seat and start getting creative to move the latch out of the way while turning the key. 

If you can't "turn" the key to unlock the seat, then follow others suggestions to press down on the seat.

There is a second problem that can happen, the seat lock cable can come out of it's "u-shaped" mounting point at the latch, under the seat. Symptom is you can easily turn the seat key lock, but nothing happens. 

If you want to prevent this problem, after you get the seat off, put a zip-tie around the latch cable to hold it in the cable's  "u-shaped" mounting point under the seat (just in front of the seat latch). You will understand when you look at it.

Here's a pic of a cable that's popped out of it's mounting point

Also be careful what you put in there. 

I stopped to help a cyclist with a flat tire one day and tried to pop my seat off to get to my flat kit. Seat wouldn't come off and the key was not coming back out. One of my Rok straps migrated and jammed up the works. 

 

We were both stranded on that mountain until things finally jiggled loose. 

Thanks guys.  With your help I got it open!  You are all SO helpful!

Rick