Are you using the Mixed Reality Toolkit in your projects? Are you using a beta version of Unity or for some other reason cannot use the master branch release? This tutorial shows how to create your own Unity package that you can re-use in your projects.
Create a Unity Package (.unitypackage) from the Mixed Reality Toolkit github repository.
I needed to create a package from the Mixed Reality Toolkit for Unity 2017.2. Here are the steps to do that: