Check your owner's manual for the radio before you go through all of this...the former owner may have jotted down the code in the section that details this. Also, my Jag I used to own had the same kind of security, and the code was taped to the inside of the engine hood, so that the technician who serviced the vehicle could reenable the radio after connecting up the battery.
Otherwise, you need a special tool to insert into the horizontal slots on the faceplate. Doing so will allow you to remove the radio. You may be able to buy the tool at the dealership, but I'm not sure how much it costs. You can get by with using a substitute, but you WILL do permanent cosmetic damage to the faceplate...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car