How do you use youtube downloader?

Ok my cuz downloaded it on to our computer and i dont know how to use it all, and i want to download the songs on to my mp3 player whenever i open it is says enter in url from the youtube video i want to download, but when i enter the url in it says it could be found or somthing like that, Can someone tell me step by Step about how to use it, Thank to u can get 10 extra points if you also tell me step by step how to give people 10 points- i dont know how to Thanks you very mucho- IF YOU HELPED AND YOUR ANSWER HELPED ME :)