From Forum Nokia Wiki

Default email: Default email account information is stored
in KCRUidMceSettings repository using 
KMceSettingsDefaultAccount key. Value stored using that key 
is the id (TMsvId) of the default email account. 
Active idle messaging plugin: Active idle messaging plugin 
information is stored in KCRUidMceSettings repository using 
KMceSettingsAIMessagePluginFolder key. Value stored using 
that key is the id (TMsvId) of the mesaging entry that the 
plugin is connected to. 

const TUid KCRUidMceSettings = {0x101F8F28}; const TUint32 
 KMceSettingsDefaultAccount = 0x00000001; const TUint32 
 KMceSettingsAIMessagePluginFolder = 0x00000004;

Usage:
1) Include Central Repository library:

LIBRARY CentralRepository.lib

 2) Include Central Repository header:

#include <centralrepository.h>

 3) Set Default email account and Active idle messaging plugin: 

CRepository* repository = CRepository::NewL(KCRUidMceSettings); 
 repository->Set( KMceSettingsDefaultAccount, 
 idOfNewDefaultEmailEntry ); 
 
 repository->Set( KMceSettingsAIMessagePluginFolder, 
 repository->idOfNewDefaultEmailEntry );