Mockups is not a RAD (rapid application development) tool. For each screen you want to design, you have to create its own mockup page.
e.g. Clone your page, make the second tab active, design the content.
Clone the mockup page again, make the next tab active and design its content.
If you want interactivity, link the just created pages over the tabs link properties.
Does this make sense?
(Edit: I just saw, that you are referring to myBalsamiq, I don't use this tool and my answer might be wrong in this context. Sorry for that.)