I have more music / videos than I have space so I have to manage it manually: so I look in my list of "Artists" or "Playlists" & when I click on the box to select it nothing happens ... I don't get the "tick" that shows I've selecetd that artist / playlist ...
I can do it if I plug in my iPad but not my iPod! - which I find strange!! Same artist / playlist .... !!
If you are manually managing contents, the only way to add content to the iPod is manually drag and drop it onto your iPod under Devices in the left hand pane of iTunes. See here for more information on manually managing your iPod's contents.