Adding videos is very similar to adding a picture.
If starting a thread you will see the video icon ( film strip ) or if adding a video to a message click the preview button.
You will see the video icon in the menu bar ( film strip ).
Click the film strip and you will see the small box open ( see below )
Now you need the code from youtube ( or other supported video host )
So right click on the embed code ( it must not be disabled ) select copy
Now right click on the box in ( image 1 ) and paste the code into it click ok.
wait a few seconds and your video will be displayed.
click send.