It's a little confusing because the "Multiplayer" button is for LAN games.  The real Multiplayer button is "Ironclad Online".  So, click the Ironclad Online (ICO) button and register and ICO account and a player name.  You'll then login and go to the Lobby.  To see available games click "Join Games".

Note that your best bet for finding games is during European and North American prime time, between 3 pm U.S. EST and Midnight U.S. EST.  Also, weekends are better.

Welcome to the online multiplayer community.  (Just make sure you've played it for a couple weeks in single player first and have a good understanding of all the races and their strengths and weaknesses because the human competition will be tougher than anything you've ever experienced in the single player game.)

you need to update your game first

I have a CD key on the back of the manual that came in the box. I assume that's what it is referring to.

which one is the cd key. theres one in the middle, and there are ones in the right and left corners in a white box. is it the middle one or the right or left?

Great!  Welcome to the online multiplayer community.

First you need to create a player name.  So <ADD PLAYER>.

After that, highlight the player name and then click <USE PLAYER> (I think).  That should take you to the ICO lobby chat channel.