Hmm...I simply turned it to the full 200mm and shoot it with the Nikon SB-900 on camera. I am not too sure if I can get any closer if I enabled the macro mode on the Nikon D90. But here's my first attempt to shoot close up with the Nikon 18-200mm lens.
1/60 second, F/5.6 at 200mm. VR Mode - Normal.