As of version 3.1.0 , the configuration files for the Payment Importer are located in the Database and not the client.

If this is an upgrade to 3.1.0, please see the information in the 3.1.0 Upgrade article.  

In order to see the database for Payment Importer, go to the RTLImport database and

SELECT connectionstring FROM rpscontrol 

If this needs to be updated, correct it using SSMS.  The connection string needs to be in the following format

Data Source=[Server Name];Initial Catalog=[Tax Database Name];User ID=[UserID];Password=[Password]

Example on sqldev01 (Without username and password):

Data Source=sqldev01;Initial Catalog=Smithclone2;User ID=[UserID];Password=[Password]