Before you join a game you must log in to the Battle.Net servers using an existing account. If you do not have an existing account, or need to create a new account for some reason try the "Make an Account" tutorial.
When you get to the Character Selection Screen select the character you wish to use for the game and press the "Enter" key or click on OK button with your mouse.
This is the create/join screen. You can either create a game or join and existing game from this screen. To select which it will be click on either the "Create" or "Join" button (lower right-hand side of the screen.)
To join an existing game enter the "Game Name" and "Password" into the appropriate boxes.
After entering the game name and password (if any) press the "Enter" key on the keyboard or click the "Join Game" button. [Note: don't click the "Join" button. Even though it is showing on the screen. It is not the correct button to press.]