Checkout
http://www.cisco.com/en/US/products/sw/voicesw/ps5520/products_configuration_example09186a008037f2a9.shtmlhttp://www.cisco.com/en/US/docs/voice_ip_comm/cucme/admin/configuration/guide/cmeover.htmlOnce you have configure the Unity to work with the CME just setup the forwarding to the AA extension on the unity express. This is part the config example above.
Download the pdf from the first link above. The AA setup guide is on page 18.
Good Luck!