In order to unlock this, you will need to take items from 255 CLEO drops in the DLC. CLEO drops occur after waves 2,3,4,5,6 in increasing number of drops.
After Wave 2 - 4 Cleo drops
After Wave 3 - 4 Cleo drops
After Wave 4 - 5 Cleo drops
After Wave 5 - 6 Cleo Drops
After Wave 6 - 6 Cleo Drops (though i had a sew waves with 7 drops)
In order to get credit you only need to take ONE item from the Drop. And this can be done if you are the first one to the drop, or taking an item from a drop that someone else already looked at.
I got my achievement when I took the last item from a CLEO drop that someone had left behind. (one of those dam hammers after wave 4... really, what are we going to do with so many hammers?) Therefore,you can pick up, add to inventory and then drop afterwards, these will count.