Pretty sure I just explained this somewhere the other day but here goes again. This is one of two ways and this is the way I do it.

Open youtube. Find the vid you want. Highlight then right click and copy on the "www.youtube" thingy.

Once you do that, come back to this page, go to "reply to thread", then you should see a row of little boxes above where you will reply. Find the "insert video" box (second from the last) and click on it.



Once you have done that another box thingy will come up.

Right click and "paste" your youtube link into this little box. and then click "ok" and "Submit reply". Then your vid should appear (hope I didn't make that to complicated. I suck at explaining stuff lol)