When you get to the part in your post where you want to install the link, click on the globe with a link in the toolbar right above the text box.
It'll open a window where you paste the link. Put the link in there then press OK.
When it returns to your post entry area, there will be a part highlighted. Just type the wording you want shown in that area.
When you're done, the text box will show something like this (using your previous link).
[.URL="http://video.google.com/videoplay?docid=4151819015985998477&hl=en"] YOUR WORDS HERE[./URL]
(I added the . before the URL= so it displays what you would see).