Anton,
Thanks so much for your reply. I got it working on my own, but I really appreciate it. Could you tell me what you mean by two scenes?
and could you tell me what you mean by a "temporary" box?
I'll reupload my model on this thread as soon as I finish the other folding leg. My end plan is to use a modified trailer jack to power the scissors up and down. The trailer jack will be driven by a parts of a broken cordless drill.
I made some progress, so I thought I'd share in case I can help someone, or in case I forget. 
I made a piston along the inside green frame.
there is a small white dowel between the green frame and the end of the yellow component. I grouped the white dowel and the piston together.
the hinge and the yellow frame are grouped.
I used the "Joint Connector" tool to join the green frame to the dowel.
I used the "Joint Connector" tool to join the hinge to the dowel.
I set the sketchyphysics properties of the green frame to static.
Thank you to the creators of Sketchup and SketchyPhysics, it's extremely helpful.
Through searching I found a few scissor related questions on this forum, but they did not discuss the problem I'm having. Right now I kind of feel stuck.
My setup:
Sketchup 2014, Sketchyphysics 3.5.5
Please see my attached file. I'm designing a scissor lift stand for my bandsaw.
Here's a general idea, but mine will be battery powered to raise and lower it.
https://www.youtube.com/watch?v=gWh3PMyWwpI
the power mechanism I'm working toward can be found by searching youtube for "stryker ambulance stretcher"
The yellow component has a hinge that's working correctly, I'd also like it to slide following the piston joint toward the center of the green component. The white component also hinges correctly.
If you run the SP simulation, the wheels of the yellow piece will fall, I'm trying to get the top of the yellow piece (the end furthest from the wheels) to slide.
Once I get the yellow piece hinging and sliding, then I'd like to add a third hinge where the center lines of the white and yellow pieces are on the same plane. I haven't gotten that far yet, so that hinge is not present yet.
This link may help, but I still couldn't figure out how to make it slide.
http://sketchucation.com/forums/viewtopic.php?f=61%26amp;t=6671
Any help is greatly appreciated.