In order to enable Pay By Mail, set the payment method to "Yes" in the Pay By Mail section in Magento. Also make sure to set up a skin and use the HMAC key in the configuration for Magento.
In order to use Pay By Mail on Magento 2, you can create an order here:
Sales >> Orders >> Create new order >> Select (/create) customer >> Select store >> Select product >> Select "Pay By Mail"
For now our product Pay By Link is not yet supported on Magento 2.