Click the Text Tool button in the toolbar. Click anywhere on the Drawing Canvas and start typing. To define the width of your text box, select the Text Tool from the toolbar and draw the box anywhere on the Drawing Canvas with a click-drag-release, then start typing. Text box height expands automatically as you type. To change text style settings, click the text box and open the blue properties menu.
Text boxes behave like any other Gliffy element, and can be easily moved, resized, and rotated.