This one is only worth doing if you have loads of Dropbox space, or not much music. If you have a lot of music, iTunes Match is a better bet as it will get expensive to use Dropbox for this. However, if you have less than, say 300 songs, it’s a good trick to have your music library available on all your machines for free.
All you need to do is start iTunes whilst holding Shift (on a PC) or Option (on a Mac). iTunes will then ask you to choose the location of your iTunes library. Select your Dropbox folder and you’re good to go! Remember to do that on each of the machines you want to sync your music with.