When you post a message on the forum, you will notice a box called Boardcode above the message box. 

Click the video icon (third from right) and the video options will appear.

The easiest way is to just add the URL of the video in the URL field and hit Insert Video.

Another way is to choose the provider (YouTube, Google Video etc) from the drop down box. Then add the ID and click insert. The ID can be found in the video URL. For example, in a YouTube URL http://www.youtube.com/watch?v=ABC123, the ID is ABC123.

You don't need to but you can add the dimensions of the video in the Size, Width and Height fields if you want to make the video bigger or smaller.