I've never had to do that on mine, it was already set up. But if you go to Settings>gereral there should be a tab that says Siri. Touch that and make sure it says on and 'raise to speak'
It ought to be where you're looking under settings, general, you normally need to switch it on and set various bits. If it isn't showing then I'm not sure what to suggest, in the general, about section what does it list the model number as?