coopster - 1:00 am on Apr 2, 2011 (gmt 0)

Well, it's not the exact same wording as the message you are receiving but if this is it ...

}elseif((!$user_data->ID) ||get_usermeta($user_data->ID,'activate_key')!==$_GET['activate_key']){ 
$errormsg=__('Your activation key is not valid for the plan!','wwm'); 
echo '<div class="wwm-errormessage" >'.$errormsg.'</div>'; 
}