what mates i have to use in solidworks to get a prismatic joint in simscape(simmechanics)

Import a multibody model into the Simscape Multibody environment. You can import CAD, URDF, and Robotics System Toolbox models. Use the Simscape Multibody Link CAD plug-in for Autodesk Inventor, Creo the STL files obtained during export of the CAD assembly have been used to create this example.

I do it by making two planes coincident. I mean the two planes which their intersection is axis of movement. for example you should make top and front planes of one part coincident with planes of the other one.

i have given the limit-Distance between the the two links. Yet i am facing problem while importing into SIMSCAPE i m not getting the prismatic joint.
Ps: check attached file.

Use "Coincident" mates for positioning ("Width" mate of you added tolerances).
As for limiting the movement, "LimitDistance" mate can do the trick.