As I understand it, MasterMap is as product designed specifically for GIS software (see here). As mentioned before on this forum, AutoCAD Map is NOT a fully fledged GIS product, it is an intermediate application sitting between GIS and CAD. As such, you can't expect to take full advantage of all the data provided as part of the MasterMap data set - for that, you will need a "proper" GIS application and not a CAD application with some mapping stuff tacked on.
From OS website: Autodesk Envision 8 has a free extension allowing the viewing of topographic layers from .gml files, the use of OS MasterMap address files for feature searches and the production of summary statistics.
I guess there may also be some 3rd party bits you can add but I reckon that will always be a compromise (more here).
See MasterMap CAD FAQ for more.