Move the contents of the enclosed plugins directory, Refeed-Plugin.php and magpierss/ into WordPress' wp-content/plugins directory.
In the WordPress administration interface, navigate to the top-level Plugins tab. Change the status of the reBlog plugin to "active".
Navigate to some other part of the administration interface, and you should see an "Install reBlog" button appear in the upper-right-hand corner of the screen. Press it, and you will be informed that reBlog has been installed.
Installation creates a new table in the database. If you see the warning "reBlog Plugin Not Installed", you will need to ensure that your database user has the necessary Create privileges on the WordPress database.
The reBlog plugin is now installed, and the button in the administrative interface will say "Fetch reBlog". Fetching new items is performed manually.