This feature is not typically recommended because it is an extra step people have to take in order to submit their registration or donation.
The purpose of the captcha is to prove it is a real person by having them enter a series of letters and words to make sure it is not a bot submitting the payment. We haven't had clients have trouble with bots in the past, so this is why we don't recommend enabling this feature.
Simply edit your campaign, then click on the Layout Tab. Look at the bottom of the listing to show your Submit Button. Hit the Edit Button. You'll notice an option to Enable Captcha which will display the spam checker with your submit button.