How do I create a circular room in ReluxPro?

Relux does not support true arcs as room shapes, but there are a few options:

There is a Semicircle option in room construction that sets the walls as a series of straight edges.

Selecting Polygon from the room type drop down menu allows you to draw a circular arc as a series of straight lines.
This can be quite tricky.

If you are CAD competent there is faster and better method:
Using a 2D CAD package, create a Circle as a Polyline
Import the DWG or DXF file into Relux and use the Create Objects from Lines icons in Relux to set the polyline as the room outline.
This gives you a very fine stepped series of straight lines as the room outline in the project.