You can add payment methods by requesting them from your Customer Area (test or live). Make sure you are at merchant account level, as payment methods can not be requested from the company level. Then go to Account > Payment methods, and click Request payment methods.
To request payment methods, you need the Manage payment methods role in the Customer Area. If this is not available for your, please reach out to your admin user.
For a complete list of payment methods, see Payment methods overview.