Private Message

For implementing the extension private-mesage (http://www.yiiframework.com/extension/private-messaging/) the sections configuration and examples not clear. I added the configuartion code in config/main.php. But after that these sections not clear to me

In order to use module you should specify User model that is used in the application MessageModule::userModel.

If necessary, specify relations for Sender and Receiver MessageModule::senderRelation and MessageModule::receiverRelation.

'receiverRelation' => array(
    CActiveRecord::BELONGS_TO, 
    'MyUserModel',
    '',
    'on' => 'MyUserModel.my_custom_id = receiver_id'
 )

Where the user model specify? and specify relations for Sender and Receiver MessageModule::senderRelation and MessageModule::receiverRelation ?

thanks