Chapter 15 Unified Threat Management for FortiOS 5.0 : Web filter : FortiGuard Web Filtering Service : Enabling FortiGuard Web Filter
  
Enabling FortiGuard Web Filter
FortiGuard Web Filter is enabled and configured within web filter profiles by enabling FortiGuard Categories. The service is engaged by turning on the Web Filter profile and selecting a profile that has FortiGuard Categories enabled on one or more active policies being run by the firewall.
There is also a system wide setting for the enabling or disabling of FortiGuard Web Filter that is only in the CLI.
config system fortiguard
set webfilter-force-off
The two options on this setting are enable or disable. The syntax of the settings name is “force-off” so in order to enable FortiGuard Webfilter you have to choose disable for the setting and enable if you want to turn it off.