Captcha Plugin Setup

I installed and activated the Affiliates Captcha plugin. but can not get it to display on the registration form.

According to the instructions on the Github page, one of the requirements is [Captcha](http://wordpress.org/plugins/captcha/), but when following that link there is the following notice:

“This plugin has been closed for new installs.”

I am using Ninja Forms for my affiliate registration and referral forms, and would love to implement captcha. Currently I am just using a text field with a math question where the user has to input the correct answer to solve and complete the form.

Any way to get Captcha working on my forms?