I noticed that if I opened the preferences directly after having started the program, without loading my project, the selection was not greyed out. So instead I stayed in 3D-icon view, added a single symbol, went back into the preferences, still not greyed out, changed to 2D-view and hit OK, then the following error message popped up.

"Sweet Home 3D must exit now because of a fatal error in 3D rendering system. Do you want to save all modified homes?"

Next time I start up the program, the selection is greyed out directly from start.

Apparently there's a bug somewhere here.

I'm running Win XP on a HP NX7400 laptop, using Intel Mobile 945 express chipset.