Run the installer which will install a SETI@home control panel. You will then have to restart your Mac.
Once your Mac has restarted you will get a login screen that allows you to set up your account and set your preferences. You now have a SETI@home screen saver and application. When either is running, your Mac is crunching data! Watching the screen saver or the application is actually fun, at least to Mac Observer Staff. :-)
Once you have created your account, head to the Team Mac Observer web page. Click on the "Join This Team" link and sign in. You will probably have to have the site e-mail you a password that they generate. There is a link for you to do so on Team sign-up page, and it is e-mailed instantly.
Allow your Mac to crunch data when you are not using it. All of your data will be credited to both your account and Team Mac Observer's account.
Visit the SETI@home web site to track the team's progress and your own!