When using the Add Map Service tool in ArcGIS for AutoCAD, when selecting secure ArcGIS Server services the following error is returned.
"The request failed with HTTP Status 401: Unauthorized".
This error occurs when using Windows Users and Groups security in ArcGIS Server.
When using SQL Server Users and Groups to administer security for ArcGIS Server, no services are returned when connected.
If security is not enabled for ArcGIS Server and the above error is encountered, make sure Anonymous Authentication is enabled for the services directory in Internet Information Services (IIS). For instructions to enable Anonymous Authentication, see the Knowledge Base article titled, "The request failed with HTTP Status 401: Unauthorized" in the Related information section below.
This is a known limit with the current build of ArcGIS for AutoCAD (build 200).