iPod won't sync with new library??

So I figured out how to create a separate library for a separate iPod by holding the shift button when I open iTunes. taht's all done. Now I've plugged in my iPod, & hit sync, & nothing happened. Then I remembered I had to find the 'music' button under my ipod button & fill it from there. But it seems to only be recognizing my old iPod lists, as they are all listed underneath "music" under "gateway users iPod" (new computer is an HP) There is no "autofill" button under teh library. So how do I get the new library on the old iPod??