UItabbar item not showing storyboard reference
This answer is pretty late, but I had an similar problem. This may be helpful for others who find this post later.
When using a UISplitViewController there is an issue for me adding the item in storyboard which can be solved with a workaround.
UITabBarController -> StoryboardReference -> UISplitViewController
You can't add an UITabBarItem to the UISplitViewController
Solution / Workaround:
Add an UITabBarController within the UISplitViewControllers storyboard and assign the UISplitViewController as one of the tabs.An UITabBarItem will be added to the UISplitViewController. You can delete the unnecessary UITabBarController. The UITabBarItem will be kept.