Creating and editing text with the Text tool
Use the text tool to add a label to an object, to edit the label on an object, or to create a new shape with text inside. 
Click a shape to start editing its text. While you are in text editing mode, the top ruler becomes a text ruler, and you can use standard Mac OS X text editing commands (for example, you can use the arrow keys to move the insertion point around).
Click or drag in an empty area of the canvas to create a new shape with settings particularly suited to containing text. A single click creates an object that expands horizontally to fit the text you enter; a drag creates an object of a specific width. Remember that the objects you create in this way are not fundamentally different from other shapes; they just start with certain settings in the Text inspector which make them convenient for containing text.
Click a connection line to add a new label to it. This label is just like the object created when you click an empty area of the canvas, except it is attached as a line label.
Text inside objects can be found by Mac OS X's Spotlight search feature, in case you need to find your OmniGraffle diagrams that contain certain words.