Tabbedpane in UIBuilder

[NetSapiens]

I've been playing around a bit with tabs, and I think I see how they work.. the only thing I cannot for the life of me fathom is how to actually make the tabs in a given window.
I'm pretty sure, that if I could crack this, it would be very possible to make some nice tabbed functionality.

So.. Has anyone been able to expand a tabbedpane in the UIBuilder? (as seen in Journals/JournalsQuest/TabPane) It has a + next to it, but I can't seem to expand it.