The issue is that I am not able to map the LookUp Field with a value from the CSV as the Lookup Field is not displayed for Mapping. Tried Import Wizard, Data Loader, Workbench, Jitterbit ... None of them display the Lookup Field for Mapping? Is there an issue that Salesforce does not expose the Custom Lookup Field in Account Object to be mapped?
I tried both Insert and Upsert options and the Custom Object has an External Id Field defined as well...