If you're having troubles embedding a youTube video make sure on you're using the old embed code.
On youTube click the embed button and then there should be a series of checkboxes. The bottom one should say use old embed code, make sure that is checked.
Or just do what dsisters pic showed... It's been a long time since I had a problem using the embedded media tool.
(ie copy the url of the youtube vid (from the url bar in your browser) and paste it into the box in the embedded media window.