Hi alexa! I managed it yesterday so I shall try and pass on this new-found skill .... you first need to 'disable rich-text' (at the bottom of your comment box) and then you copy and paste the 'embed' bit, from the youtube page, into the comment box.

Then put the rich-text back on and voila! Should be sorted. It'll show a box with the flash 'f' in it probably, but when you post the blog it'll show the youtube vid.

Hope that's right ;) i'm hopeless with IT usually!