Have your VIN# handy and call your local Mitsubishi dealer. To enter
the code you use the first four station selector buttons. Hit the
first button until the first number appears, the second button until
the second number appears, the third button until the third number
appears and then the fourth button until the fourth number appears.
Then hit the TAPE button.

You only get three tries to get it right then the radio locks up.
HOWEVER, there is a way to reset the radio to get another three
attempts.  With the key in the "Accessory" position leave the radio on
about 45-60 minutes. Later turn it off and back on and you will be
greeted with the prompt to enter the code again.

See a dealer, they need to pull the radio, call the manufacturer with a
number off the back of the radio and they'll get a code to enter on the
radio to reactivate.