Learn here how to register for the site in English. Please follow the instructions given in the screenshots below.
As for your location, please choose “その他(海外など)” which means “Other (outside Japan)”.
After the final step, a page appears stating you have to confirm the registration within 24 hours by clicking on the link the service sends to you via Email after this registration process.
Do so and you are ready to go!
You can find more information on Nico Nico Douga on Joi Ito’s Web Log in Fumi Yamazaki’s excellent article.