Problem With Installing The Private Messaging Module

Hi, i am trying to configure and install the private messaging module as found here: http://www.yiiframework.com/extension/private-messaging/ and I am getting the following error after installation:

Fatal error: Class ‘CActiveRecord’ not found in .../public_html/protected/config/main.php on line 54

i added this code to the config file:

‘modules’=>array(

‘message’ => array(

‘userModel’ => ‘User’,

‘getNameMethod’ => ‘getFullName’,

‘getSuggestMethod’ => ‘getSuggest’,

‘receiverRelation’ => array(

CActiveRecord::BELONGS_TO,

‘User’,

‘on’ => ‘User.id = receiver_id’),

‘senderRelation’ => array(

CActiveRecord::BELONGS_TO,

‘User’,

‘on’ => ‘User.id = sender_id’),

),

User extends CActiveRecord

Can anyone explain to me the proper file extension for this module as i’m a bit confused. Do I rename a file somewhere?

we added the files contained in the controller file to or controller files etc, and run the sql scripts etc

Thanks for any assistance you can provide

Kind Regards