I have removed the old YouTube software from this forum, and installed new software. It works better, and also handles Vimeo videos. To use it, copy the URL (
http://whatever) of your YouTube or Vimeo video. Look for the YouTube icon in the row of icons just above the row of smileys that you see while composing a message. Press this icon, and insert your URL between the bracketed "youtube" and "/youtube" that appear. To the right of the YouTube icon is the icon to press for Vimeo videos, and it works the same way.
To copy a video's URL, highlight it and type Control-C. To paste it in a message, ensure your cursor is in the right place, and type Control-V.
I hope this isn't too confusing.