I had a similar problem with my wheel. Under my desk, about 2.5inches in, there is a bar running along which is essential for support. It's about 5inches 'tall', so made it impossible because it was set back. Anyway, I just cut through it - well, I got my Grandad to! You just need to get a wood plane, and shave off from the bottom of the desk the 0.5 cm or so needed in the area you will put the wheel. If it's more than that, get a handsaw from somewhere if you don't have one, and cut into the desk! It sounds drastic, but you've got no other option.
But your desk must be darn thick