From the Pen tool Tool
Options palette, choose Line Segments 
.
Click and drag from the line starting point to the location of the next node. Release the mouse button.
To extend the line: click, drag, and release again.
Use the Pen tool with the Line Segments option to draw straight lines.
To draw using the straight lines:
From the Pen tool Tool
Options palette, choose Line Segments .
Click and drag from the line starting point to the location of the next node. Release the mouse button.
To extend the line: click, drag, and release again.
Note: To draw lines at fixed angles (45 degree increments), press and hold the Shift key as you drag.
Drawing Lines and Custom Shapes
Drawing Curved and Straight Lines