It would appear that the node itself does not possess an input pin for the class of actor component you wish to attach. For whatever reason that is the current design. If you wish to make a feature request for them to add it, you can do so. Or you could just click on the node, and modify it directly as Nathan suggested. Another alternative would be to make your own node in C++ that simply references the existing node object, but adds another parameter to the function (sort of a wrapper function). That way, the new function will have the additional input pin that you are expecting.
Here is a link to a quick guide on creating custom nodes: