Plates don't have openings. You can create a mesh of plates where the lack of plates in a region creates an opening.
RISA-3D now has the ability to define "wall panel" elements where you tell the program the dimensions of the wall and draw in some openings and RISA will interanaly create a mesh of plates to form the wall and opening. Is this what you were trying to do? If so, this technique is only allowed for nominally vertical walls and cannot be used for slabs or sloped / inclined walls.