It's off by default, because it drastically lowers game performance. PerformanceFu and Option House both have options to toggle profiling on and off. Note that turning profiling on or off requires a reloadui, and I think both of those addons will trigger one automatically when you use them to turn profiling on or off.
Like I said, use PerformanceFu or Option House and look for the option to enable/disable profiling. For PerformanceFu it's probably an option on the menu that comes up when you right-click its FuBar display (can't remember for sure). For Option House, bring it up by typing /oh then go to the performance tab and hit the button on the lower-right.