At some of the files in my queue cannot be chosen the _search for alternate location_ menu item. it is grayed out.
If you want to search for alternates, the file must have a TTH root. That's the long wtring of random characters that pops up in search, and what happens when you first add your share.
Manually search for alternate resources by filename and size, copy the most appearing TTH in search results and add it to the proper file in Queue.xml... then you'll be able to search for alternates.