The site is removed from the Most Visited. Hold the right mouse button to pan around your aircraft, and as the on-screen guide says use Shift+mouse wheel scrolling to change the size of the decal (you can manage some pretty gaudy effects by scaling decals up to cover most of the plane), and Alt+mouse wheel to rotate it.
After selecting a decal the cursor changes to a paintbrush, and you can position the decal on the aircraft.
