how do you add a border around text in omnigraffle?
If you mean how do you outline each character:
In OmniGraffle Pro, select the text you want to outline with the selection tool, then tap and choose “Convert Text to Shape”. With the shape selected, use the “Stroke” settings in the Object Inspector to apply different kinds of outlines to the text. Convert Text to Shape is an OmniGraffle Pro feature.
If you mean how do you outline the entire text box:
Select the text box itself, then use the Stroke inspector to set a stroke on the text box. This applies to all OmniGraffle Standard and OmniGraffle Pro.