It's easy if you know, made a quick tutorial for you:
1. Click on the Share button below the video.
2. Check the boxes indicated with the red arrows.(using old embed code is the most important.) (HD only if possible of course.)( 560*345 is a nice size that will fit on this forum.):
3. Done that? Copy the html code in the big box.
4. Click on the flash button in the fancy GUI reply box:
5. Paste embed code in the text box that appears in the pop-up and click on add video.