Determine where Homebrew would normally download the archive file to, so you know where to put the manually downloaded file. Open a Terminal window and enter brew --cache. This will give a path on where the archive file would normally be saved to before getting installed.
Copy/move the archive file from Downloads to the folder path you found with the brew --cache command.
(From http://mygeekdaddy.net/2014/12/05/how-to-install-a-local-file-in-homebrew/)