Here is how to post videos.

EDIT: I just added a new module that makes it automatically convert a link to a video. so no more of this trouble. all you have to do is copy and paste the link to the youtube/googlevideo page.

This old way will still work though if you want to change the size or somtehing.

First of all copy the embed code for the video from YouTube or wherever the video is hosted.
The code should look something like this

<object width="425" height="355"><param name="movie" value="http://www.youtube.com/v/EkAlaq6UMP4&rel=1"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/EkAlaq6UMP4&rel=1" type="application/x-shockwave-flash" wmode="transparent" width="425" height="355"></embed></object>

When you are posting a new topic click the link under the text box that says "disable rich-text."
Then paste the code into the plain text box. and hit Submit.