Before you can post a video in our forums you first must have your video uploaded on a video service such as Youtube or Google Video that provides an embed link.
Step 1: To post a video you will need to embed the code into the html. Open the rich editor and click on the "HTML" link, as shown below

Step 2:
Copy the embedded video code of the video (for example, youtube video) you would like to post.
Step 3: Paste the embeded code into the html editor and click on "Update"

Step 4:
Click the Preview tab to make sure it looks ok and than post.