Slot or Slider Constraint

[hugou]

I'm having some trouble working out what constraint to use and how to apply it to a simple mechanism. I have not used Mechanisms a lot and and don't know how to define the feature I have modelled.
Attached is a screen shot.
What I would like to do is add a contraint to the slot, about the hole in the lower step which will allow a pin to travel up the slot in the link bar before folding the upper step in vertical and parellel to the lower step.
Hope that makes sense.
Do I need to be using a slot, a slider or what to allow linar motion and rotation along the slot? How do I apply the relavant constrain, do I need a sketched curve, and axis running parallel to the slot or what?
Thanks in advance for the help.

 

[hugou]

Just for clarity, in case it's unclear what I'm trying to achieve, the lower step rotates through 160° and along it's way picks up the upper step which *should* then rotate through 90°.
The upper step will havwe a stop which will prevent it from going past horizontal.
The lower step overlaps the upper step and the link bar fits between the two.
Cheers.

 

[hugou]

update: I have worked out the slot constraint, I needed to add a datum curve.
Now Ihave another problem, I can't drag the mechanism with the ‘drag components’ handle as I get the message in the dashboard “You may not drag a model belonging to the ground body.”
What does this mean and how to get around it?
Cheers.

 

[aafuni]

I haven't really worked with the mechanism features, but I believe you need to define a component as the fixed component, or the ground body. If you have it properly set up, one feature will remain grounded as the other features move relative to it. You probably have components grounded that should be free to move and that is causing the error.

Sorry if I am off on this one, but I hope I was of help.
-AAFuni