Sorry - please forget what I said about the D gates. I checked my old boarding passes again and the last 2 times I took a KL flight to AMS from FCO it left from Gate B13 (Must have been thinking of another flight). The most recent was on 7th March this year. If the same is true for you then you will need to double back on yourself after visiting the lounge in D - about 10 minutes walk.

If it is B13 again, it is at the far end of area B on the left, next to the Mercedes Benz Cafe.