Not sure if you've found your way to it yet, but on the Dell I'm running, you begin tapping F2 on boot up to enter Setup. On the first page of setup, scroll down to System Security and hit Enter. Scroll to Chassis Intrusion and use the arrow keys to move left/right. Reset to Enabled. Hit Enter. Hit Esc. Select Save Changes and Exit. Hit Enter.
"'Tis an ill wind that blows no minds." - Malaclypse the Younger