There are many different codes for these radios. What you will need to do is contact a dealership to get the correct code for the radio.
One item you will need is the information from the label that is located on the top of the radio. To remove the radio is really quite easy.
1. Gently pry the trim plate from around the radio. This can be accomplished by taking a flat blade screwdriver and carefully prying up and out at one of the bottom corners. Once you see it pop out, repeat this on the other bottom corner. The bottom will be out and you can then pull the top away from the dash to remove it. There are two small plastic tabs on the bottom of the bezel, and two metal clips on the top that secure the trim to the dash. Very easy to do.
2. Once the trim is off you will see 4 Phillips head screws that attach the radio to the dash. Remove them one at a time (be careful not to drop them into the dash like I do every time I do this LOL). Once the 4 screws are removed simply pull the radio towards you out of the dash. You do not have to remove it completely to see the tag, it is located on the top of the radio on the drivers side.
3. Write down the numbers on the tag. One is the model number that is in bolder lettering that starts with either an MR or MB. The other is the serial number.
4. Call practically any Mitsubishi dealership and ask for the service department. Tell them you are looking for a code for your radio. They will more than likely just need the serial number and within seconds will give you the code for it.
Mitsubishi dealers are generally great to deal with, and I have found them to be extremely happy to give you the code for free.
Once you have the code, reinstall the radio, turn the key on, turn the power on to the radio and enter the code pushing the #1 station button until you get the first digit, then the #2 station button until you get the 2nd, the #3 station button until you get the 3rd, then the #4 button until you get the 4th.
The next step depends on your radio. Once you have the code showing on the radio you will either have to push the "program" button to enter it, or the "Cassette" button to enter it.
You will have 3 chances to do it right, and if you fail and the radio says "lock" on the display do not panic. All you will need to do is leave the key on for one hour or so, and the "lock" will disappear, and "code" will return.
Good luck, and treat your dealer kindly, they are there to help you.