If the class in question already has an IBPalette, you can create the instance the same way you would a button. See HowToCreateAButton.

If your class is a subclass of NSView, you can created an instance of the superclass (NSView or any subclass with an IBPalette) and then change it’s class to your subclass. For example, if you want to instantiate a MyButton, you would create an NSButton and change its class to MyButton.

If you want the object to be at the top level of the Nib, select the class you want to instantiate and choose Classes > Instantiate MyClass.

HowToProgramInOSX