Under the Guides>quick start tab, there's a link to the rule book as well as to introductory videos, and under the Guides>game rules tab there are the game rules (there's a link in the quick start tab to the game rules tab). Also under guides, there's a new player tab and an FAQ tab. That seems pretty straightforward, and if you don't like to read, well...online diplomacy is probably not for you then.
The point about the forum being byzantine is fair. That's why I listed it as the least likely to happen or work. It definitely has a learning curve to navigation if you've never used it or one similar to it before.
Edit: Seward beat me to it.