Okay, here's what you do: double click on the video screen so that you go to the actual YouTube site. Over to the right of where the video plays, you will see the uploader's name, the tags for the video, the URL and something called the Embed code. Copy that code.

Then, go open the Livejournal site itself (I don't think you can do this through client software, at least you can't do it through mine) and pull up the screen to post an entry. Note: you can't do this at "My LiveJournal" [http://www.livejournal.com/portal/] If you're there, click the "More Options" button to go to the Update Journal page [http://www.livejournal.com/update.bml]. You will see, just under where you type in the subject line, two codes, one for "Insert Image" and one for "Embed Media." Click "Embed Media" and insert the code you've copied into the box that pops up, and then click "Insert." Hope that helps!