Try the following:
Turn on radio. The radio display shows "SAFE".
Press the P.Scan and RBDS buttons simultaneously and hold until "1000" shows on the display.
Release the buttons. Do NOT press these buttons again, otherwise the radio will consider "1000" to be the code.
Use Preset buttons 1 to 4 to enter the four-digit code number from your radio card. Preset button 1 corresponds to the first digit, Preset button 2 corresponds to the second digit, etc. Each press of the button increments the number by 1.
Once the code shown on the display is correct, press the P.Scan and RBDS buttons simultaneously and hold until the word "SAFE" appears in the display, then release the buttons.
The radio should unlock and be usable shortly afterwards.
If an incorrect code is entered, the word "SAFE" appears, first flashing and then continuously. You can repeat the above steps to unlock the radio once more, the number of attempts will be shown in the display. If an incorrect code is entered again, you radio will be locked for about one hour.
After one hour - the radio must stay on and the key must remain in the ignition lock - if the display goes out, the you can repeat the above procedure to unlock again.
The cycle - two attempts, one hour lock-up - still applies.
If this doesn't work then its possible that the radio has been replaced at some point so the code in the book no longer applies
in which case I'd suggest removing the unit from the dash and getting the serial number off it as once you have a serial number then If you remove the radio and retrieve the Radio serial number off or the side (starts in AUZ) then call Audi’s customer assistance to speak with a Audi Customer CARE Advocate. Call 1-800-822-2834 (AUDI), Mon-Fri, 8 am to 8 pm, Eastern Time. If you tell them your Vehicle ID number and the radio serial number they will ask you some questions to verify the vehicle is registered to you and then they should provide you with the code for FREE. This typically takes a couple of days.