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Copy the retaggr folder and its contents to your wp-content/plugins directory, then activate the plugin.

The plugin will add a new retaggr section under the Settings menu. You need to configure one thing here - your retaggr Site ID. This you get from retaggr.com - just log in and add a site. You will then have your Site ID, which you can copy and paste into Wordpress. On the retaggr site you can set all the options regarding moderation, and which features you want enabled.