Problem projecting image since Mavericks upgrade

As a teacher I use my MacBook Pro daily to project images using a dongle from my computer to the projector. When I plug in my dongle the image projected is the top bar of my computer (time, battery power, etc) but the image is simply the ocean wave/ripples of the new Mavericks system. I have gone to display preferences and image mirroring is on, but I can't get it to display anyting else? I even bought a new dongle thinking that was the issue, but it isn't. Please help! My teaching is about 90% using my computer so I am drifting at sea right now. :-)