I've notice a few people having issues with posting & embedding videos on these forums, so I thought I try to help out.
When you copy the YouTube link into your message if the link shows as blue you need highlight the link and click on the "Unlink" button (just left of the "Image" button, looks like a chain-link with a minus symbol (-) under it). When you unlink the link it should turn black like regular text, now when you submit the post the video should show up as a embedded video. Best of luck...
Example (random video):