If the formspec is properly formed, it should just open on right-click. However, note that the documentation for formspecs may be wrong. On the wiki it shows examples of "context" being used for the current node's inventory, and in the lua_api.txt for 0.4.4 it says:
"context": Selected node metadata (deprecated: "current_name")
I'm not sure whether to interpret that as 1.) "context" is deprecated and "current_name" should be used in newer versions, or 2.) the opposite. I DO know that in 0.4.4 stable "current_name" works while "context" does NOT (and when I used "context" I got the same behavior you are experiencing: no formspec opened on right-click). I haven't updated the wiki on this because I'm not sure whether the behavior in the 0.4.4 release is newer, or if it is old and the documentation needs to be updated to represent the newer behavior on the development branch.