Turn on the User-defined properties check box on the Properties tab to override the solver variables selected by MotionView.

Solver Variables panel - User Defined tab

The User-Defined tab will allow you to create the string which the solver input deck will use to invoke the user subroutine.  This tab also allows you to optionally select a specific .dll file that contains the compiled user subroutine, as well as an optional user subroutine name.