If you want to show the registrations of the TrainingEvent, you should at least place the listview inside the dataview. If you have done that you will probably be able to do what you ask because then it is possible to choose registration over association from the TrainingEvent as you describe.
At least... I think that's what you're trying to do?
The connector in studio pro is only available if you need certain object. By this I mean that you will only see entities and attributes in the connector if you have selected a Dataview or an Input widget, for example. The connector is context dependent.
The Connector pane displays elements that can be connected to the currently selected element. For example, when a button is a selected, the Connector shows microflows that you can drag onto the button to connect them.
thanks a lot for your considerate answer, by which I find the connector after selecting the list view.
but there is no entity under certain entity like the learning path shown, only entity under MyFirstModule. is there something I missed?