If you want to post a Youtube video;
- Go to the video on Youtube.
- Check the address bar. The URL will be something like www.youtube.com/watch?v=XXXXXXXX&feature=g-all-f
- Copy the XXXXXXXXX part, i.e. after v=, before the ampersand (&), if present.
- Paste that code between a [youtube] tag, as demoed above.
Also, you cannot post widgets like that.