Private Messaging Module

I have just downloaded and tried to configure the private messaging module: http://www.yiiframework.com/extension/private-messaging/

I am getting the following exception when trying to access the module: Alias "message.MessageModule" is invalid. Make sure it points to an existing PHP file and the file is readable.

This is the relevant code in my main.php configuration 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'),
),

Has anyone else had this problem before? I would really appreciate the help. Thank you for your time