The program uses pythons urllib to get page sources, and to download the video will use another site keepvid.com which provides a list of direct download links for the video in different formats using the youtube url of the video. So if you paste the following string...
"http://keepvid.com/?url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3D" plus the youtube id "kpCJyQ2usJ4"
In there I think it's the parameter kv_url used in the kv_info and kv_ds functions which contains the string.
The entire page source:
A small part of the script which I think is where it is: