Creating and Viewing a Document - Adobe Illustrator CS6 on Demand - page 102

Click the General list arrow, and then select a measurement from the available
options.
Click OK .
Important
If the Rulers are not visible in the active document, click the View
menu, point to Rulers, and then click Show Rulers.
Work with Rulers
• Show or Hide Rulers. Click the View menu, point to Rulers , and then click
Show Rulers or Hide Rulers .