Mapping a field in Connector for Dynamics AX 2012 > CRM 2013

We have Dynamics CRM 2013 integrated with Dynamics Connector to AX 2012. The integration is set up using the latest Connector version and running OK in default configuration.

We need to a new mapping in Customer Service to Account map. The mapping should be as follows:

AX field: ABC Code > CRM field: Account Classification

I can locate the ABC field in the Connector mapping wizard, but I get a validation error:

I am not very familiar with the AX data model, but I assume you cannot access that field in AX directly like this (even thought the wizard lets you believe that). AFAIK the field is located in a different table (DirOrganization?). There are several functions in the Connector wizard for accessing data, but I cannot find any documentation about them. What would be the correct way to map this field?