I am very surprised that no member has answered your question.
I have not been on this site for some time but i think that there are two ways to post a video.
(1) if it is a short video then you can add it the same way as a normal photo.
(2) you post the video on Youtube and then past the link in the text section.
There is the quick site guide that should help.
As i have said I have not been on this site for sometime so things might of changed.