First go to the video you would like to embed and copy the video code: http://www.youtube.com/watch?v=[color=#FFFF00]CtZKL74LgMg[/color]
In the forum click on the "youtube" button and you will see this [youtube][/youtube] now paste the code you copied in the middle, like this: [youtube][color=#FFFF00]XXXXXXXXX[/color][/youtube] The code should replace the X's.
That will embed your YouTube videos in the post.
Hope this helps!
