The better way is to drag & drop them onto the Library icon in iTunes. The exclamation mark means iTunes can't find them. See How to transfer music to my iPod (& iTunes) about this & alternative methods. The songs that are already in iTunes, can iTunes play them ok? If not, you may need to double-check the iTunes music library location. Go to iTunes menu bar, Edit>Advanced. Sorry the location of this setting depends on which version of iTunes you have.