You may need to set up automatic syncing for the iPod in iTunes.
First, if the iTunes sidebar (along left side of iTunes window) is not visible, from the menu bar, under View, select Show Sidebar.
Find the iPod in the sidebar, under DEVICES, and select it. There is a bar of buttons near the top, starting with Summary. Click on Music next to Summary. This is the iPod's Music screen, where you tell iTunes how to sync songs to the iPod.
NOTE: When you set up automatic syncing, any songs on the iPod that are not in your new iTunes library will be lost.
Check the box for Sync Music. You can choose to sync Entire music library, if your iTunes music library fits on the iPod. With this setting, your entire iTunes music library syncs to the iPod. Otherwise, you can choose to sync Selected playlists, artists, albums, and genres. Then, select (checkmark) the playlists, artists, albums, and/or genres you want on the iPod from the lists below. If you want newly added songs to sync to the iPod, they must be part of your selection. For example, you can add them to a playlist that you have selected for syncing to the iPod.
When you click Apply, the songs from your selection sync to the iPod. Going forward, changes to your iTunes library (based on your settings on the iPod's Music screen) sync automatically to the iPod, the next time it is connected (or when you click Sync).