Walls are referred to as segments in FPSC. To place segments (walls):
1. Select the "segments" tab from the library
2. Click the "Add New Segment" Icon (top of library)
3. Click on the various Segment Library files to locate a segment you like the look of, when you are happy with your selection double click the icon of your choosing.
4. The new Segment will now appear in the Library window.
5. Click the newly added segment, then place you mouse over the 3D grid. Hold the Left mouse button and drag the mouse, you will hopefully be creating some walls and corridors by doing this.
6. To add just walls ( minus the floor ) click the M key, pressing the R key will rotate your cursor to paint different segment face directions.
Good Luck!