Drag custom view onto window in Interface Builder?-Collection of common programming errors

Interface Builder doesn’t add (or allow the addition of, now that IB Plugins are derelict) Custom Objects to the Object Library. You can only drag in existing/preset objects, but you can have IB instantiate them as instances of a subclass as long as they are instantiated the same way.

E.g. Drag in a custom view and set the class to MyCustomViewClass, or drag in a scroll view and set the class to MyCustomScrollView, or drag in an object and set the class to CustomObject, et cetera.

And that is the preferred way of using custom controls in Interface Builder.