Enter Things mode. Right click the second sector, then on the blob that appears. From the window that follows, expand the "Editor Things" menu and choose "3D mode start." Click OK, then press W - you'll see your map in 3D, and can navigate with E, S, D and F. Point the camera at any wall and left click. Press Tab, select Brick1, then mousewheel click the repainted wall to copy its texture. Right clicking on others will paste. Texture the room.