On your computer, check if you’ve changed the place of your iTunes library. If the library isn’t in the default place anymore, Nokia Link won’t be able to get any info from it. The default place for the library is usually [local hard disk drive] > Users > [username] > My Music.
If you want to copy the music that you have in iTunes, but you don’t want to change the place of the iTunes library, then choose the folder that contains the music you want to copy instead of choosing iTunes from the programs list.