How to get the Fixed # and Code number for 91-Maxima

I have a second-hand used car -- 1991 Nissan Maxima. There are several buttons beside the front door lock. It used to lock and unlock the door without a key.

However, I don't know what is the passcode for that buttons. The manual say there are a "fixed number" and "code number".

Anyone know how can I find the two numbers? How can I use this digital entry system.

Thanks.

this might be helpful
www.apro.org/_Keyless_entry-26219276-358-a.html