there is no need for any credentials if you want to receive Idocs. Setting up an RFC Server to receive Idocs needs only this 3 SAP Properties: GatewayHost (usually your SAP Server), Gateway Service(usually sapgw + Instancenumber of the SAP System) and ProgramID. You can find a tutorial how to setup the ProgramID following this link:
More Details how to receive an Idoc :
http://help.theobald-software.com/ERPConnect-EN/default.aspx?pageid=example-receiving-an-idoc
Best
Ali