I assume you want to do something like this:
http://www.brompton.co.uk/ourbikes/g...tem.asp?i=1857 (not my picture). I love how it looks too.
If you look closely at the picture, there is at least one hook visible, but they probably used two hooks per bike. The hook you can see is right behind the Brompton sticker by the seat post. The other hook is presumably by the front hinge.