Older versions of the Entegra software will not have an AUX tab, the system will have a setup tab as sown in the photo below. 

Photo #1

To retrieve the serial number of the MFD, power on the MFD, accept all messages and go into flight mode. Navigate to the Setup tab and hold down the L1 and L3 buttons (left side of the MFD) to get into Maintenance Mode. In Maintenance Mode select System Info Photo #2, The serial number should be in the middle of the page, see photo #3.

Photo #2

Photo #3