Changing Slide Order

I have created a slideshow by simply flagging the key photos and using the created 'flagged' event as my show. This is for a talk so I don't want to automate the projection via the Slideshow software because the duration of each projection will change according to questions etc and I need full photo on the screen. Or is there a way of using the slideshow whereby the slide changes only by a mouse-click or, better, via the remote sender?
How do I change the order of the photos in the flagged event?
Dragging doesn't work and the 'sort' menu has the 'manual' option greyed out.