Setting up an outlet property will be the same for any object - just create a property set to missing value, for example:
propertyyourProgressBar : missing value
Properties with this signature will show up in the Interface Editor, where you can make a connection to the UI object. For example, from the Connections Inspector for the App Delegate, all of the outlet properties are listed, so it is easy to just drag a connection from there to the desired object.
After an outlet property has been created and connected, it will represent the class and instance of the object, so you can use the various methods of it and its parent classes (all of the following are equivalent):
yourProgressBar's startAnimation_(me)
startAnimation_(me) ofyourProgressBar
tellyourProgressBartostartAnimation_(me)