It is against the YouTube terms and conditions to download a video from YouTube and put it into your Keynote presentation. But if you have the author’s permission you are allowed to stream a video from Youtube. The only way I know to do this is using a free application called ‘LiveSlides’
You can download the live slides application from here:
1. Open your presentation in Keynote.
2. Open Safari and get the Video URL
Open the video in your web browser – go to Youtube, open the video and right click and select ‘Copy URL’
3. Run LiveSlides
Launch ‘LiveSlides’ and paste the URL (Apple – V’ )into Liveslides. Liveslides will put the link for the live video into Keynote.
Now when you play your keynote presentation, it will download the Youtube video live from the Internet and play it.