I am having two thoughts for you. First, the EWS, or Electronic Immobilization System, which has already been mentioned, would prevent the car from cranking.
You can use a digital multimeter to diagnose this properly. Have somebody turn the screwdriver to the start position, and you should get battery voltage on terminal 50 on the starter solenoid. (There are two skinny wires and two fat ones; terminal 50 is the skinny one on the bottom.) If you do not get voltage when turning the key, your EWS is tripped, and you need INPA diagnostic software and an EDIABAS cable to connect to the car.
It sounds like you already powered terminal 50 directly to test the starter. I am not 100% sure that will work on these cars, because it looks like the ground path on the other end of the solenoid going through a transistor in the IKE (instrument control cluster). I'm getting this from ELE-179 in the Bentley manual, btw.
Assuming the starter test works, or if you get battery voltage on terminal 50 when you turn the screwdriver to the start position, then I suspect your starter motor is bad.