One other thing.

When you have assembled your post with video. Look at the bottom of the box you post in and you will see 2 small boxes below the "post icons" section. One will say "submit reply" and the other says "preview post". If you preview your assembled post with video added and you've done it correctly, then the video will be visible. Sometimes there is no "preview post" box but one that says "go advanced", but basically it does the same thing.