Proxy Real IP is a simple plugin that sets user's IP addresses correctly when running WordPress behind a load balancer or other proxy.
Installation is simple.
/wp-content/plugins/directory or the
/wp-content/mu-plugins/directory if you're running a Multisite installation and want to force load the plugin for all sites.
/wp-content/plugins/, then activate the plugin through the 'Plugins' menu in WordPress.
There are no settings for this plugin in the WordPress admin area. It should just work out of the box.