im trying to build a holographic app using unity newest download (personal) w/ visual studios 2017

I got a book called HoloLens beginners guide I just cant complete the first project Hello World!. I have finished all until I come to build settings its fine but than it sends the correct info to visual Studios I see that there's no error so why I cant connect my HoloLens because highlighted in red can't find mobile sdk and can't find mobile file in red... So i go to visual studios 2017 newest update to download all DLC I than try repairing the visual studios but I still can't connect my HoloLens.... I have to get help its frustrating when its no other way than my ideas..