WordPress.org

1. Extract the files from the compressed "zip" file (note the location of the extracted folder/files)
2. Upload the contents of the bns-login folder (from above) to the /wp-content/plugins/ directory
3. Activate the plugin through the 'Plugins' menu in WordPress

-- or -

1. Go to 'Plugins' menu under your Dashboard
2. Click on the 'Add New' link
3. Search for bns-login
4. Install.
5. Activate through the 'Plugins' menu

Read this article for further assistance: http://wpfirstaid.com/2009/12/plugin-installation/