There is a push button LED, probably with a plastic cover on it next to where it says "battery in trunk." The LED only tests the diagnositc module because that's related to emission problems. Once you have found the push button, turn the ignition on without starting the car. Then hold the button down for three seconds. Whatever fault you have will be represented by the number of flashes. It will only give you one code at a time, so if you have more than one code, you will have to repeat the process.
The way you clear these codes is to hold the button down for three seconds, let it flash whatever fault code (I would also write it down), then hold the button down again for 10 seconds. Then, immediately turn the ignition off for 15 seconds.
For every code on your diagnostic module you will have to repeat the process.
There is a code that is common with these cars and that is code 19. Check out the article in this link (scroll down). You cannot reset this code.
http://www.k6jrf.com/MB_S500.html