The Music URL
Right, first of all, you gotto go to a website where you can download MP3's. For example www.mp3-center.org
Then, search on the song you want (and hopefully, it'll be found by the search engine).
Then press the songs name.
When you've done that, you'll see a "Download now" link, right-click on it, and select "copy link" or something like that.
You should then have a link like this: http://culture.dyu.ac.kr/lec/files/Linkinpark%20-%20In%20The%20End.mp3
Just paste that into the "Play song from URL" on Line Rider, and you're all set!
Edit: Typo
--
Never trust a computer you can not throw out a window 
here's my link but i can't hear music
http://ghostofmars.free.fr/RUSH/Linkin%20Park%20-%20One%20Step%20Closer%20(1).mp3
Oh yeh very helpful my computer just freezes...
http://images.google.co.uk/imgres?imgurl=http://faqsmedia.ign.com/faqs/i...
Im Having Problems!
What Kind Of URLs are you suppose to put on the URL Bar