Here is sample code to get OdBrBrep from manifold_solid_brep. Code:
#include "Entities/StepGeometricRepresentationItemMD.h"
...
OdDAIObjectId idManifoldSolidBrep = pDatabase->getModel()->getEntityInstance((OdUInt64)440); // Get manifold_solid_brep, here it is taken by handle for "known" instance.
OdDAI::OdCompoundPtr compound = pDatabase->getCompound(idManifoldSolidBrep); // Get compound that composes geometry for it.
OdStep::OdStepGeometricRepresentationItemMDPtr representationItem = compound; // Convert into RepresentationItemMD
OdStep::OdStepModelerGeometryPtr modeler = representationItem->getModeler(); // Get modeler.
const OdBrBrep &brBrep = modeler->bodyContainer().brBrep(); // And finally access OdBrBrep.