LCY is fairly easy - just get the shuttle bus to Liverpool St, and you can get a (slower than King's Cross, but not bad) train from there.

But, as said before, STN is by far the best airport for Cambridge. There are direct trains and coaches, and it's only about 30 miles away. Assuming you're coming from Germany (given your profile), you should be able to get a decent Air Berlin flight into STN. It's really a lot less hassle.