Interface builder is pretty good, but one thing I found confusing was how you add new tab views to a tab bar controller.
Instead of clicking on the Item and dragging it do a new view controller, the way to add a new tab is to click on the tab bar controller itself, and drag it to the new view controller you want to hook up. A new Item will automatically be added.
Just select relationship view controller when you do the drag and you’ll be fine.
Oh ya and if you are creating a tab bar controller from scratch, just take your regular view controller and go Editor -> Embed In -> Tab Bar Controller.
Here’s a link that helped.