The simplest way is to first find the video you want to add on YouTube, and get the "embed code" there. It shows up where you're viewing the video.
You go to "edit Hub". At the right-hand side of the page there is a gray shaded area that allows you to select whatever capsules you want to add. You select "video" and will get a message that it has been added at the bottom of your Hub.
You then go to the empty capsule and select "edit". There you will see where you can enter the code you got from YouTube. Remember to hit "save" after you've entered the code.