classes_def.xml error

You need something like this in a classes_def.xml file to load the new data products:

<lcgdict>
<class name="art::Assns<anab::T0,simb::MCTruth,void>"/>
<class name="art::Assns<simb::MCTruth,anab::T0,void>"/>
<class name="art::Wrapper<art::Assns<simb::MCTruth,anab::T0,void> >"/>
<class name="art::Wrapper<art::Assns<anab::T0,simb::MCTruth,void> >"/>
</lcgdict>

You also need a classes.h file in the same directory of the form

//                                                                                                                                                                                                                         
// Build a dictionary.                                                                                                                                                                                                     
//                                                                                                                                                                                                                         
// $Id: classes.h,v 1.8 2010/04/12 18:12:28  Exp $                                                                                                                                                                         
// $Author:  $                                                                                                                                                                                                             
// $Date: 2010/04/12 18:12:28 $                                                                                                                                                                                            
//                                                                                                                                                                                                                         
// Original author Rob Kutschke, modified by wes                                                                                                                                                                           
//                                                                                                                                                                                                                         

#include "canvas/Persistency/Common/Wrapper.h"

// data-products                                                                                                                                                                                                           
// lardataobj                                                                                                                                                                                                              
//#include "lardata/Utilities/AssociationUtil.h"                                                                                                                                                                           
#include "canvas/Persistency/Common/Assns.h"
#include "lardataobj/AnalysisBase/T0.h"
#include "nusimdata/SimulationBase/MCTruth.h"

//                                                                                                                                                                                                                         
// Only include objects that we would like to be able to put into the event.                                                                                                                                               
// Do not include the objects they contain internally.                                                                                                                                                                     
//                                                                                                                                                                                                                         

template class art::Assns<anab::T0,simb::MCTruth,void>;
template class art::Assns<simb::MCTruth,anab::T0,void>;
template class art::Wrapper<art::Assns<anab::T0,simb::MCTruth,void> >;                                                                                                                                                   
template class art::Wrapper<art::Assns<simb::MCTruth,anab::T0,void> >;

Go back to $MRB_BUILDDIR and rebuild after zapping.