How do I add music from my computer to my iPod?

I am unable to locate any ways of adding already downloaded music to my iPod. I am not seeing options to select a device nor do I see any type of Source Pane. I already have music on the iPod that was added a while ago. Due to a computer issue (restored to factory settings), the music is still on the device, but I do not know how I can manage the content. I do get the option of erasing the iPod and olny adding the new music, but I do not wish to do that. The only options I have on the left side of iTunes are Library, Store and Playlists. I have created a playlist, but I do not know what to do with it now.

Also, is there any way to pull up the music currently on the iPod for management?

Thank you - I am a technology moron!