There are two ways to tackle this: modify an existing NavBar; create you own, as you've discovered..
For the first:
Double-click on the Navbar to open its Properties.
At the bottom left, click the Ungroup to edit graphics button.
You can now use the Text tool to enter a line break by pressing the Enter/Return key.
The problem you will face is the best way to deepen the button and then adjust the text to fit.
I use a mixture of Edit Inside, the PAge & layer gallery and setting the view quality back to outlines.
When happy, right click on the button and pick the create Navigation Bar... option.
You can do the same for a Stretch button.
For the second;
Personally, I find it easier to create a text box and build up a shape around that for the MouseOff layer.
When I have that right a copy it into the MouseOver layer and change the attributes.
If you have Designer, you can also make the button shape stretch to the text entered, both horizontally and vertically.
You then have to soft-group the layers (not forgetting MouseDown).
Finally, clone and enter the required text labels, remembering to ensure each button have the same number of text lines.
That said, I avoid the built-in NavBar capability and use MouseOff/MouseOver buttons to open Layers that open other Layers for deep sub-menus.
That way, my "buttons" can be anything I want.
I also prefer pure CSS3 Navbars, so I create these in a Placeholder:
http://line25.com/tutorials/how-to-c...dropdown-menu; http://codepen.io/philhoyt/pen/ujHzd are some examples.
You need to understand HTML unordered lists and hyperlinks and also CSS to style the links.
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