You are OK and correct to be concerned what that does and how it works. The main point I want to make again is that you should NOT be messing with it if you aren't getting closed loop. You have no way to know where it will end up when and if you ever get it in to closed loop, and if it isn't in closed loop, nothing in changing.

I'll try to explain it again;
If you look down in the hole where the anti-tamper ball or plug was, you will see a hex shape, that accepts an allen wrench. The other guy seems to confirm it is a 3mm. What that is is sort of like an extension down into the airflow meter and it is spring loaded in the "up" position. You insert your allen and press down on it, the lower end then goes down and engages into an adjuster screw. Sometimes you need to fiddle with it a bit to get that lower part to engage into the actual adjuster down below.
The only way to know is by trying to see if the crack is messing anything up. If it is you can buy a new adjuster tower.
First thing I'd check in regard to the fixed operating mode (ie not going in to closed loop) is the overvoltage protection relay (check fuse).
Gilly

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Gilly is correct. Engine has to go into the closed loop operation first before you make any adjustments. The crack in the Lambda tower could have some effect on your idle but more unlikely. To check it out, run the car and spray some carb cleaner around the crack to see if it fluctuates or effects the performance. If not, you're probably OK. If it's not going into closed loop, replace O2 Sensor and check OVP relay fuse (I replaced my OVP relay thinking this was an issue but just wasted my money on it as the biggest changes came from O2 sensor and water temp sensor then lambda adjustment). I also replaced Coil, Voltage Regulator (won't effect idle) and rotor and cap. Recently I noticed my spark plug wires arcing to the engine case! This was causing a rise and drop in idle and lack of power. Replaced the suspect wire and problem gone.
To make the Lambda adjustment, put the 3MM hex into the hole and make sure it seats in there ok. Then push lightly until you feel the screw spring pushing back upwards and turn it until the two catch together. Then make your adjustments left or right. If you push down to hard your car should try to stall as this drops the throttle plate down and richens the mixture quickly. Again, it won't work right unless the air housing is on and the air temp sensor is working (which, it probably is).