Yeah, almost all of the action is on the Diplomacy expansion server.
You said you couldn't get it to work? Are you clicking the button that says "Ironclad Online"? The "Multiplayer" button is for setting up direct connect to IP games. The public online multiplayer where you would meet strangers is the "Ironclad Online" button. Also, you need to have the most recent version of Sins installed to get online. So you need to have registered your CD key and updated your game with Impulse, which is like a type of STEAM. You can get it here: http://www.impulsedriven.com
The current version of regular (Vanilla) Sins is v1.191, so at the bottom right of your screen, it should say "v1.191" if you have the most up-to-date version.
Now, the big issue for you is whether or not it makes sense to spend $20 on getting the Diplomacy expansion, which requires the purchase of the Entrenchment expansion. You really might be better off waiting for 6 months (or however long) until the upcoming stand-alone Rebellion game is released. It's going to cost $40 and you can no longer receive a discount for owning Diplomacy.