Upload 'geomap.php' to your plugins folder, usually 'wp-content/plugins/'.
Create the folder 'wp-content/geomap' and upload at least one map image (JPG, PNG or BMP) into it. One is provided with the distribution, but you can use your own by placing it in this folder.
Activate the plugin on the plugin screen.
Configure with your desired options using the options which will now appear at the bottom of the Geo Info Options sub-menu.