How can i get iTunes to sync my iPod?

previously i have plugged my iPod into my laptop just to charge as my music was on another computer. now i have moved my music onto the laptop so i want to "Erase and Sync" my iPod to my laptop. however, i have previously selected the option not to erase and sync and selected the "don't ask me again" box so the pop-up/window with the choice to erase and sync doesn't appear. my iPod comes up on my desktop but not in the iTunes source list - what should i do?