When you decide to post somethin...
you go down the site and you click in a little square which becomes big when you actually click in it..
then you see there two little squares...
one is Post quick reply and the other one is go advanced...
so you click go advanced..
then the new page is opened and you see up this place where you can write somethin..
a little square where is a sign of youtube.. you click on it and then you have to write in it an url of wanted video, you write an url and press ok
then submit post...
then others can see the video that you wanted for them to see...

i hope you understand!!!