It's that round thing on top of the injection pump. It has a metal shield around it to prevent you from tampering with it. If you feel the need to tamper there, you will have to carve that shield away from it.
Best advise I have read on that part came from Engatwork, he noted on the best running MB's he has seen, the ALDA was left alone. Not always true I suppose, but the point is there are a lot of other parts around that one that very strongly affect the way the ALDA functions, and that affect the fuel flow just as well as that part. So take your time to sort it out and get it right the first time.