The parting plane is the contact surface of the stationary and moving halves of the mold that separate when the mold opens.
A newly imported model must be correctly oriented within the mold before an analysis can be run. When they are first imported, models are usually oriented so that the cavity side of the mold is oriented towards the sprue; however, the mold parting plane is in the X Y plane with the sprue entering the mold from the positive Z direction.
You can change the orientation of the model by using the rotate tool to rotate it about the center.