Directly on the Youtube video page, look at the address bar and copy the last part of the video URL.
Go to the site editor and click 'Insert' from the main toolbar.
On the left sidebar, click 'Youtube' in the 'Widgets' section.
The widget will appear on the page. Double click on it to bring up the editing window.
Paste the copied code, then click 'Apply'. You can view the video by clicking 'View my site' in the top right corner.