I got my unique access code while doing a job in Mafia Wars. Some players got it when fighting and other when they’re robbing. Each player has the chance to get an access code. You must keep it a secret and should only be entered on the Central Mainframe Access.
You can write it down or did what I do and copy/pasted it in Notepad and saved it. The popup message states that you must enter this code on the home page each day to receive a bonus. I don’t know if you could get a new code if you lose your current one.
To go to the Central Mainframe, just go to the home page. You’ll find the Central Mainframe Access in the Daily Checklist. When I entered my code today I got +3 Attack. I don’t know if this is random or constant with every player today. Can you tell us in the comment what bonus you got today to know if this is random or not?