Is your car not under warranty? Quite a common fault on the C's. Also get the uprated metal linkage if you dont want it to break again lol, dont really know how to change it though mate so cant really help you.
i replaced my gear linkage last week. was out of warrenty so had to fork out 99 quid for the full linkage, not the repair kit. easy enough to do. garage will charge you about an hours labour. i did it myself without any hassle