Did you press Button 5 afterwards to enter the code?

The code for the given serial number is, indeed, 3339. You can check it for yourself using:

www.pumapeople.com/decode.php

(Your given serial number is M052355, not MO52355, by the way)

How did you obtain the serial number? The radio could have been changed at some time during the car's life if you haven't owned the vehicle from new. It's also possible that the audio manual isn't the one that came with the car when new. To make absolutely sure you need to read the SN from the label which is stuck onto the outer casing of the radio. You will need a set of radio removal tools in order to be able to do this.

Or you can try pressing Buttons 1 and 6 together as you turn the radio on. This should produce a scrolling stream of numbers, one of which should be the serial number.