I swear I've answered this question a dozen times, so I'm capturing to refer people in the future. The tree control in my OmnGraffle Wireframe Stencil is a godawful hack that uses tabs and arrow characters. Copying and pasting the arrows, however, screws up the formatting. OmniGraffle just doesn't make this kind of manipulation of text all that easy and I haven't found an ideal solution, but if you want to learn how I do it, read on.
To use the tree control presently, you utilize the tab stops and insert arrrows from a character viewer. Type CMD-R to show rulers and tab stops in paragraphs of text. Then Option-Tab to move between tab stops. 1 tab for first level arrow. Insert arrows from the character viewer.
The video below shows how to :
1) Enable the character viewer in OS X System Preferences > Language & Text > Input Sources.
2) Use tab stops to Option-tab your text to create hierarchy.
3) Use OS X's character viewer to insert arrows.
View it in full screen to see it.
I'm using Option-* to make the bullets. Like I said it's terribly awkward, but that's how I make do.
Since using paragraph formatting in a text element is unnecessarily difficult, what might be an easier way to manage is to use a table and copy/paste the arrow icons if you find the above technique a pain.