As an ERO, your submitter information will differ from a regular Business Taxpayer.  Since you have an EFIN, that item will be used during the filing of your returns with the IRS.  Place this number below the line in the e-File Submitter Information dialog.  You also need to add your own 5 digit self select practitioner PIN.  Your EFIN, Practitioner PIN and a 5 digit self select taxpayer PIN will be used as your signature to file these customer returns.