WordPress.org

1. Copy the gengo directory in the zip file to wp-content/plugins/
2. Activate the plugin on the plugin screen.
3. Add the languages you wish to use on the configuration screen. The first language you add will be the default language, and all existing posts will be marked as being written in this language. For English, use the locale en_US, which is WordPress' default.
4. Change your wp-config.php file by hand so that WPLANG is the locale of your default language.
5. If a Gengo localisation .mo file is available for your language, also add this to the /wp-content/languages/ directory.

DON'T FORGET STEP 4!

Upgrading

1. Deactivate ALL plugins before doing anything.
2. Upgrade your WordPress to the latest version (if available)
3. Upgrade Gengo files under wp-content/plugins/gengo/
4. Login, and activate the plugin.