1) Copy the URL of the video you want to show. You can find the URL in the address bar of your browser or next to the video screen in YouTube.
2) Push the YouTube button (

) on top of the editor screen, next to the Tab and Chord button.
3) Something like this pops up:
|YOUTUBE| |/YOUTUBE|
4) Paste the URL in between |YOUTUBE| |/YOUTUBE|
5) Done!