Adding files to events in Media Pro catalog

Events seem like a useful way to index images. An event can be created by clicking the triangle next to the event heading, but it isn't clear how to get images into an event. I have tried going to the folder with the events I want to add, selecting all the events in that folder and then dragging to the event. That seemed to work once, but after that the files couldn't be dragged.
It would be nice if there is a menu setting somewhere that allows images to be selected and to have the selection added to an event.
Is there a preferred way to add to an event and can hierarchies of events be created?