Any plugin is to be uploaded using FTP into the plugins folder and then activated using the plugin manager.
But then this is not a plugin. This is a mod. If you would have read the first sticky post you would have been led here:
http://forum.coppermine-gallery.net/index.php?topic=28367.0 (http://forum.coppermine-gallery.net/index.php?topic=28367.0)
Where you find this piece of text:
QuoteInstallation:
Before you do anything make a backup of your files and your database!!!!!!
Unpack and upload the files in the archive. Keep the folder structure intact. Then run update.php .... done (you'll get a fatal error if you open the index without running update.php)
If you have a bridged Coppermine rerun the bridgemanager