To add text, click inside the shape (a hatchmarked border appears around the shape) and type.
To add a name and other text, you click inside a shape and type. Press ENTER to put text on a new line within the shape.
When you enter text, the shape's selection turns into a hatchmarked border (see the illustration on the left). It's significant to note this because you need to have this type of selection (as opposed to the gray-dot border) to do specific things with text, such as editing it or copying it if you want to paste it elsewhere.
You shouldn't have this border for certain actions that involve shapes, such as tabbing between them. For example, if there's a hatchmarked border around the shape and you press TAB, the text in the shape is tabbed (indented).