Ah, there is your problem! It's a MK III. The Audio text option uses a screen format that is only supported by software version V24 and above, and that is not available on a MK III.

In order to display Audio Text you need :

New CD Changer, Sept 2005 or later.
New Radio module BM53 or BM54, Sept 2005 or later.
Mk IV Nav computer running V24 or later.