Assuming that your answers to Leonie's questions reveal no problems with the installation, you need to determine if the plug-in is actually there.
Are you sure that the plug-in file is installed? You should find it in Library/Application Support/Aperture/Plug-Ins in your home directory, or in the disk's: Library/Application Support/Aperture/Plug-Ins. To access Library in your home directory, select the Finder menu item Go while pressing Option.
If the plugin is present, you may have a problem with file or folder permissions, and you need to change those on the Plug-Ins folder accordingly.
If nothing above helps, re-install.
Good luck!