This is actually a candidate for the FAQ because it comes up a lot... but apparently almost exclusively in e-mail as I couldn't find where I had directly answered it in the forum before. Odd.
Anyway... here's the deal... Bliz only directly supports up to 120 buttons on 10 bars of 12 buttons each in its action bar engine. I use that same engine in order to ensure that nUi has maximum compatibility with both Bliz and other mods.
It just so happens that being a member of Bliz's favorite class, your Boomkin is the only character that uses all ten of those action bars. For everyone except you, that bottom right bar is never used, so it's an extra bar to them. For you, it is not.
Unfortunately, there is no "other" bar I can give you there. Every other bar in the Bliz engine is either already displayed or is used by many classes for various things (other shapeshift bars, etc.).
As already mentioned, I will be greatly extending nUI's action bar functionality in time to include supporting more than the just the default 10 bars Bliz supports, supporting intelligent bars that offer one set of actions for friendly targets and a different set for hostile targets, etc. However, that is not at the top of the priority list given how much core work remains for things like the layout tools, etc.
So... unti then, I'm afraid you'll have to just put up with that bar being unusable for your Boomkin.
What people don't get is that I am, ultimately, an artist at heart. My brush has two colors, 1 and 0, and my canvas is made of silicon.