Questions about Apple malware protection

I'm running SnowLeopard 10.6.8. Earlier this year, I thought I installed Apple's automatic malware protection. In System Preferences, Security, General, the option for "Automatically Update Safe Downloads List" is grayed out. When the lock at the bottom is unlocked with your password, you can make changes to the list, but the lock always winds up locking again and the option grays out again. I'm not sure it it's working or not.

When I tried to via the Terminal following MacWorld's instructions to check and see if I was up to date(System/Library/CoreServices/CoreTypes.bundle/Contents/REsources/XProtect.met.plist), I get "no file found".

What am I doing wrong and how can I correct this to automatically update the definitions? TIA

You do nothing. Apple takes care of it for you. If you are running Snow Leopard or Lion, it is on and working and you are not involved.

This story should summarize it nicely:

OS X updates to block Revir.A as new Mac Trojan appears | MacNN

Thanks Chas. I also found an app called "Safe Downloads Version" from Mac Observer that tells you what your latest version is just by clicking a button.

You're really overthinking this. You don't have to worry about it, manage it or check up on it. It Just Works.