In IB bring up the Identity Inspector tool (Command-4) then select your custom view and in the Class pop-up choose the name of your custom class instead of generic UIView. You may want to connect it to an ivar as well. In your ViewController declare an instance of your custom class with an IBOutlet in front of it. Then go back and bring up the Connections Inspector and connect your view to the ivar by click-dragging from the custom-view's referencing outlet to the File's Owner (which should be an instance of your ViewController) and choosing the ivar name.
When your NIB is loaded it should be creating an object of that type and connecting it to that variable.