after downloading you need to run the installer first: I got my version of the plug-in from this site: http://www.apple.com/downloads/macosx/aperture/blackandwhite_geoffreyclements.ht ml
After downloading I found an installer in my "Downloads" folder.
To install, double click the file: "BlackAndWhite Installer.pkg.zip" to unwrap the package and then run the installer by double clicking the package.
Then the plug-in will be installed either for all users or for you only, either in
- System Library: /Library/Application Support/Aperture/Plug-Ins/Edit/BlackAndWhite.ApertureEdit/ or
- User Library: ~/Library/Application Support/Aperture/Plug-Ins/Edit/BlackAndWhite.ApertureEdit/
To unstall, drag the plug-in from these folders. If you did not run the installer, just remove the installer package from your downloads folder.