If unable to add custom fields

There is a conflict between Strong Testimonials and older versions of the WooCommerce Helper plugin, AKA WooThemes Updater, which prevents adding new custom fields.

In the testimonial fields editor, if you click “Add New Field” and see the message “Are you sure you want to do this? Please try again”, make sure you have the latest version of WooCommerce Helper installed (1.7.1).

You can find the latest version here.

This is a perfect example of why deactivating other plugins where possible is such an important step in isolating the problem.