Goto youtube.com, find the video you want and copy the complete URL like from the address bar.
Paste it in and hit enter after the text, preview your post to see if the video displays. If it does, then click post.
If you are still having trouble, you can try the second method.
Add [ youtube ] before and [ /youtube ] after the URL
This example is displayed with spaces included so you can see the code.
[ youtube ]http://www.youtube.com/watch?v=IDhW6ZZ1-1E[ /youtube ]
Below is the result

[youtube]http://www.youtube.com/watch?v=IDhW6ZZ1-1E[/youtube]
Pontiac Solstice