Can you point me to the video(s) on YouTube you're trying to download? There are many ways to download YouTube videos however the vast majority on there are copyrighted and so you're not allowed to use them without express permission.
If, at the bottom of the video description, it say this:
"Licence:
Standard YouTube Licence"
Then you're not allowed to use it.
However if it says:
"Licence:
Creative Commons Attribution licence"
Then you can.