If you are using the latest version of iTunes, the sidebar may be hidden. From the menu bar, under View, select Show Sidebar.
In the sidebar, the iPod should appear under DEVICES. Select it there, and you can set up automatic syncing for music to the right, on the Music tab (next to Summary).
Alternately, if you want to load music manually, on the iPod's Summary tab, check the box for Manually manage music and Apply. With this setting, you can drag songs to the iPod from your iTunes library.