The Archer Web Services Transporter is used for "Archer-to-Archer" data feeds. For the "Load Fields" button to work, Archer must be able to successfully authenticate and connect to the source instance's API to retrieve the report metadata.
As detailed in the Archer Administration II Data Feed module, if no fields appear, it almost always indicates a communication failure between the Feed Manager and the Web Service. The Transport Configuration section (found on the Source Connection tab) contains the critical connection details: the Base URL, the Instance Name, and the User Credentials. If the credentials are incorrect, the URL is mistyped, or the "Session Token" cannot be generated, Archer cannot "see" the source report's structure, and thus the field list will remain empty. Checking the connection settings is the primary troubleshooting step before looking at filters or target mappings.