Even when "hide extensions for known filetypes" is checked, AND your system files are set to be hidden, and you put in ".dll" in the search field, it SHOULD find all the dll's on the drive or folder you are searching for. I just tried it and it works for me. Explain what happens when you do a search.
p.s. If you have a relatively fast machine, you shouldn't have to use indexing. I turned it off for all my drives.