No option to sync iPod

I have my iPod connected to two computers. On the second computer I changed it so that I could manually add the music to it in iTunes, and that worked fine, however, now there's no option to sync my iPod. How do I do this? I don't want to erase all of the music that I added from the other computer, but it seems like that's the only option in order to get the music from this computer onto it.