The Add button is the one you normally use for iTunes. Unless they have changed it. How to transfer music to my iPod (& iTunes)

To get rid of the LimeWire playlist from iTunes, go to LW's menu bar, Tools>Options>iTunes>Sharing & uncheck this option. As you know, that playlist only allows you to play the songs but can't be copied or burnt from.

From memory & I will need to check this up, the import button is intended for importing playlists, not actual song files.