Hi Dimitri, the "onsrf" object snap will only be activated if you are picking the point in the 3D viewport. If you're not seeing it then you're probably picking points in one of the Top/Front/Right ortho viewports instead of the 3D viewport. When you pick points in the ortho viewports onsrf is disabled because the ortho viewports are focused on making 2D planar drawings and onsrf has a tendency to fill the entire screen with snap points at different z levels.

Some more information here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=7692.12

- Michael