Modifying Box into wedge shape.

1. Switch to Vertex selection mode.

2. In the front view, drag a selection around the top right vertex. Note that because "Select Through" is on, both the vertices (the near one and the far one) get selected.
      The info line above the front view should say "Vertices seldected:2". If it says anything else you've done something wrong, so undo (ctrl-z) the selection and try again...
   -  An aside, AC3D suports multiple undos, although there are some actions which cannot be undone or otherwise clear the undo list (saving in native file format is one of these, alas..)

3. Go to Menu->Vertex->Snap Together. The top Right 2 verts will snap equally towards each other, appearing as a single point (althought there are still 2 verts there - we'll clean that sort of stuff up later).
      The result will look a little odd...

4. Repeat steps 2 & 3 for the bottom right vertices.
      You should now have a nice, clean wedge-shape (see the BG of the pic..).

On to the fun bit...