The 'unable to locate' message means that the physical music files are no longer on your computer, or not where iTunes is expecting to find them. What 'old library' are you trying to open when trying to locate those files ? You need to search for the music files (the .mp3, .aac files, depending upon what format that you have them in) - is that what you are looking for on your computer, not an iTunes library file (.xml or .itl) ?
Have you got copies of those music files on your backup of your downloads/library ? If not then do you have them on an iPod or iOS device that you can copy them back from ?
Or what you can redownload will show in the Purchased link under Quick Links on the right-hand side of the iTunes store homepage - if you still have the songs listed in the My Music part of your library (where you are getting the 'unable to locate' message) then might show in the 'All' section there not in the 'not in my library' section, you would need to delete those missing entries from My Music for them to show there.
If the music has been removed from the store by the rights-holder then you won't be able to redownload them, nor if you've moved countries since buying them. And check to see if you've hidden any of them : Hide and unhide purchases in the iTunes Store - Apple Support.