I can add objects, but I can't rotate them. Use the keyboard arrows just moves them over a square. But I need to rotate desks, benches, etc...

For me this is a problem depending on where I am in the room. Even if I seem to have enough space if the game deems that I'm "too close to a wall" it won't let me rotate anything. Try moving it around and then rotating just to see what happens.

Try clicking right mouse button while holding an object.