When attempting to Direct Connect to the underlying database of an ArcSDE instance, as described under the “To set up a direct connection to the underlying database“ section of the Procedure here, you receive an error, such as “Unable to establish connection to provider. FDO Provider entry point function 'CreateConnection' returned NULL”
As noted in the Map 3D help (linked above), 3rd party .dll files are required for ArcSDE functionality. Those .dll files are currently only available in a 32-bit format, which is incompatible with 64-bit software.
Installation of Map 3D or Civil 3D on a 32-bit system is the only supported option for Direct Connect to ArcSDE.
Products: AutoCAD Civil 3D;AutoCAD Map 3D