In Salesforce navigate to Administration Create / Apps and click on "new" in the Connected Apps section.
Set up the App as seen below, don't forget the callback path and make sure you enter a contact email AND set Selected OAuth scope to “Access and manage your data (api)” via the add button next to the list.
Once you click on save, you'll need to grab the Consumer Key and the Consumer Secret in order to use them in the credentials set-up for the Subscription Suite.
Use this page to enter additional Salesforce credentials in Subscription Suite:
Host is SFDC URL (use X.salesforce.com and not the lightning url even if you have lightning enabled)
SFDC Username and Password
Customer Key Connected App (see screen above) = Connect App Client ID
Connected App Secret Connected App (reveal first) = Connected app Secret
Navigate to the "remote site settings" in Salesforce Settings and add your Subscription Suite domain: