To add text outside the area of a plot or graphic, first click the plot to make the resize handles visible.

Using Ctrl+drag on Windows, or Cmd+drag on Macintosh, drag a resize handle to add space for text:

Bring up the Drawing Tools palette by selecting it from the Graphics menu:

Click on the Drawing Tools palette:

The cursor will appear with a small "T", indicating that you can add text. Click where you want to add text:

Type in your text:

You can also add other annotations, such as arrows, using tools from the Drawing Tools palette to draw on top of the graphic.