If you are using the latest version of iTunes, the sidebar may be hidden. To show the sidebar, from the menu bar, under View, select Show Sidebar. With the sidebar visible, the latest iTunes works (mostly) like previous versions.
Does making the sidebar visible allow you to add the new music to your iPod?
If the problem is something else, please post back. It would help to know if you use the manual method to load songs (drag and drop songs on iPod from iTunes library) or if you use automatic syncing (by setting up the iPod's Music tab in iTunes).