How to fill Macbook screen when in Keynote, just like Powerpoint fills the screen of Windows.

Use these these settings to maximise Keynote for your display:

• Keynote > Preferences > Slideshow > Scale to fit display

• Keynote > Preferences > General > select Scale placed images to fit

• Inspector > Document > Document tab > Slide size dropdown menu: select custom, then enter the pixel size of your Mac display in the width and height entry boxes. You can find the pixel size of your display from: Apple Menu > About this Mac > Displays tab 

If you mean - Play Keynote in full screen, then use one of these methods:

• click the play button in the toolbar
• use the keyboard key: F8
• Play > Play Slideshow