Just copy the last part of the video URL after the v= in the url and paste it in the box and it will give you the embed code.
Here is an example:
Normal quality with embed code youtube gives you.
High Quality with the embed code the website gives you.
Since the video URL is http://www.youtube.com/watch?v=HgODgN--HGM
copy HgODgN--HGM which is after the V= and paste it in the box.
So if you ever want to embed a youtube video in high quality, thats how you do it.