Please follow this link to setup a Recurly account: http://recurly.com/gateway. More information can also be found here: https://docs.recurly.com/payment-gateways/recurly
Note that Recurly accounts acquired through Salesforce AppExchange will not work with this integration. This is because these types of accounts don’t have full access to Recurly’s API.
Payment Center Setup
After you setup your Recurly account, retrieve your API Key and plug it into Payment Center. To get your API Key, login to your Recurly account, click on API Credentials on the left. This should display your API Key, as shown below:
To plug your API Key into Payment Center, go to Payment Center Settings tab, under Payment Processors section, click Recurly. In the Live Settings section, enter your API Key, as shown below:
When you are ready to go live, ensure that you have checked Live and Use As Default boxes (also ensure that your Recurly account is in Live mode):