Can only add mp3s to itunes by dragging individually, not through "Add to iTunes"

Obviously this poses a problem when you have 350GB of music to transfer..

Some background: I am a former PC user who sought a technician to recover files from a crashed PC hard drive. The technician recovered all files all transferred to an external HD (unfortunately not Mac formatted) and I connected to a new 4GB Mac mini. After transferring music files to a desktop folder, I can see my entire former itunes folder but around half of all files are grayed out. Sometimes it's the band names, sometimes the album or track names. My understanding is that these are hidden files, but after using the "killall finder" in terminal it does not change anything. If I go to "Add to itunes" I can pull up the folder but none of my music appears for me to sellect. I can individually drag any files (black or grayed) into itunes and they will work - they'll populate and back up in the mac's music files.This is the kicker, though - the grayed out mp3 are now black. They appear normal. The frustrating part is I can't shift files en masse, i can't only shift them by drilling down the file tree in my music folder and dragging groups of mp3s one album at a time. It would take me months and months of doing this to get everything onto itunes. PLEASE HELP! I cannot find any other examples of this on the net!! Is there software that can make itunes "see" the files again and enable me to import all at once?