Playing a YouTube video during your presentation can be difficult switching between the Keynote and a browser window. By following these steps, the YouTube video will be apart of your presentation and will play right on your slide during the presentation.
Go to the slide where you want to insert the YouTube Video.
Across the top middle of the window, select 'MEDIA'.
Select 'WEB VIDEO'
Paste the YouTube URL into the Add Web Video Box
And now you can adjust the size of the video on your slide