Ok. Solved it..
Seems to have been a Version Problem. I installed an older Version (7.1.0) of WDK (only by bruning an ISO… found it here: https://www.microsoft.com/en-us/download/details.aspx?id=11800). Still the mentioned header ist not findable by Windows Search. But VisualDDK now just found the needed Folder itself while opening the Reconfigure Program.