You will need access to the administrator account via the web browser.
On the left hand side, hover the cursor over "NBX Messaging", then select "Auto Attendant".
From there you can add a new AA by pressing the Add button.
You will name it, use the system assigned extension number to keep things simple, and choose how many times the prompt will play to a caller before following the T/O setting on the key definitions.
Once the attendant is created, you can go back to NBX Messaging, Auto Attendant, and click on your new AA to edit the key actions and the prompt. The online help files in this section are very well written. You will also be able to define and record your prompts.
Remember to ACTIVATE your AA before leaving the screen.
You can easily test by dialing the extension number assigned to the AA when you created it.
If you have any further specific questions, feel free to post. ~ Mike