Well, there are a few methods of doing what you're asking.
1. Enter the HTML yourself.
Do this by typing
[A description of the link]
2. Type in a description of your link. Highlight the description, and then click the little earth-icon with a chain-link on it. Copy/paste the desired URL into where it asks, and you're set.
3. Click on the same icon, copy/paste the URL, then edit the description between the HTML tags where it currently just defaults the URL itself as the description.
For example:
Thor Von Clemson's Advanced Fast Hand Finger Wizard Master Class
(This is my video description)
Just highlight it, click the icon and paste the URL.
[ URL="http://www.youtube.com/watch?v=MSUz5GtZyko"]Thor Von Clemson's Advanced Fast Hand Finger Wizard Master Class[/URL ]
This is what the code should look like (without the spaces at the beginning and end) before you post it.
Thor Von Clemson's Advanced Fast Hand Finger Wizard Master Class
And this is what the final result should be.