How to enable collision on a static mesh that was converted from a BSP brush?

I converted a BSP brush into a static mesh and now I want to enable collision on the static mesh, but collision isn’t working on it at all. My character goes right through it. And I have checked that all of the settings are the same as a static mesh that I just drag and drop from the content browser. Am I missing a step in the conversion process?

Edit the static mesh, then choose “Collision” from the menus at the top of the static mesh editor.

cannot find “Collision” :S what are the menu name ?