Disconnect the blade
Begin on the driver's side of your ML550. There is a small button on top of the wiper arm; the blade itself is attached beneath the arm. Push the button to release the wiper.
Remove the old blade
Gently slide the wiper off the wiper arm. If the button has been depressed, the wiper should slide directly forward off the front of the wiper arm.
Attach the adapter
Place the adapter on top of the blade. Line up the 'v' on the adapter with the 'v' on the blade and snap it in to place.
Position the new blade
Gently push the wiper blade on to the arm as you slide it back along the arm. The blade will snap into place and the button will come through the hole on the arm with a 'click'.
You're all finished with the driver's side. Repeat the process on the passenger side of your ML550.