Settings > General > Siri > Siri On.
To use Siri: Hold the home button for 3-5 seconds and speak to it.
About Siri...
http://support.apple.com/kb/HT4992
If this is not working for you...
Try This...
Close All Open Apps... Perform a Reset... Try again...
Reset ( No Data will be Lost )
Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears. Release the Buttons.
http://support.apple.com/kb/ht1430