In a UIViewController you can implement viewDidLoad and publish a message containing the uiview or the uiviewcontroller (depends on what you want to mediate). Have a mediator listening for this message and then register the appropriate mediator.
In our iphone app we wrapped the NSNotificationCenter into a custom event dispatcher like you would find in a typical UI framework. This is how the UIKit components dispatch events that our mediators listen too. Then of course our mediators talk to the rest of the PureMVC system through Notifications.
Hope this helps