1.  Click on Tools the main toolbar.
2.  Click on Out of Office Assistant.
3.  Click on I am Currently Out of the Office.
4.  Enter the text you want to appear when someone emails you while you are away from your
     office in the upper box area.
5.  Click Ok.

Make sure you turn this off after you have returned to your office. You would follow the same process but click the option I am Currently in the Office.