Ian,

If under Windows, installing Sketchup shouldn't cause any problems.
If under Ubuntu, you should read this message .
If under Mac, I can't help you.

Check out this file. It shows you examples of:
- a triangular window in a wall
- a triangular window in a wall with an extra triangle to fill the gap. Note that SH3D has problems with the texture.
- a triangular window as part of a sloping wall.

I hope this explains how to do what I tried to explain earlier.

Hans