You don't need to.
If you have a burning need to add something to your Mac, get ClamXav.
And don't go on to questionable sites. And don't click "OK," when a screen comes on telling you that you have a virus. Because you don't. Clicking on it will only install malware. But Apple has issued a security update to prevent crapola like that appearing.
Apple released a free software update (Security Update 2011-003) that will automatically find and remove Mac Defender malware and its known variants.