To be clear. An interface function WITHOUT an output variable shows up fine in All Possible Actions dialog…
An interface function WITH an output variable does not.
I had same issue, but found solution.
First, you need to save and compile.
Second to implement a function, I had to double click on function found in left hand side under interface. It will take you to function to be implemented.
Rightclicking on EventGraph to implement it only works if you have a function without an output. If it does have one, you have to double click on function on LHS.