If you have a Keynote file follow the steps below to properly set up the slide show and begin screen sharing. 

• Open the Keynote file in edit mode
• Click on the Play tab at the top of the Keynote window
• Select Play Slideshow in Window

• The Keynote Play Slideshow window will appear and the size of the window can be adjusted by clicking & dragging the corners of the window

• To begin sharing your screen click on the Screen Sharing button ---> Application Window ---> Select the Keynote application window