I just updated to Xcode 5.0.2 and in interface builder on the lower right corner where I can drag and drop objects I don't see combobox any more. I tried using the search field below and typed combobox, NSCombobox, but nothing.
Combo boxes are available only for Mac projects, not iOS projects, so you're getting the expected behavior. If you create a Cocoa application project and type combo in the object library's search field, the combo box and combo box cell objects should appear in the object library.
I'm surprised you were able to access a combo box in earlier versions of Xcode. I don't remember combo boxes ever being available in iOS projects.