These two placeable trigger are playing for the person you on the standart maps or where on the map the MilktruckTrigger are not installed!
The triggers for the http://www.modhoster.de/mods/watermilktrailer needed to fill from the cow pasture and then me to sell him with milk.
In the shop there then buy 2 Trigger:
MilkTruckFillTriggerPlaceable to fill the trailer he has a black / white mark.
MilkTruckStartTriggerPlaceable to sell the milk he has a Black / Blue marker.
Mutatis mutandis, one should place the FillTrigger in the cow pasture and the start trigger in the Molkerrei, but that is not a requirement.
You have to klinken with the mouse exactly to the spot to trigger re-sell.