. Get the Installer - Log in to your Battle.net account. In your list of games, click the “Manage Game” button under the StarCraft II beta image to download the installer. You can select PC or Mac from this interface.
2. Install the StarCraft II Beta - Once you’ve downloaded the installer, run it and follow the directions to install the beta client on your machine. Note that your participation in the StarCraft II beta test is governed by a Beta Test Agreement that you are required to accept before you can install the beta client.
3. Run the Game & Log in to Battle.net - After the game installs, the launcher interface should automatically appear on your screen. Log in with your Battle.net account and you’ll be able to begin playing. You can launch the client at any time by clicking the desktop shortcut or by going to the Windows® Start Menu.
Any questions? Check out the FAQ.