In this Nice1 guide, you will learn how to import and set up Nice1 plugin for Unity (Windows and Mac).
Next, open up the Unity project you want to use or a blank project. When your Unity project is open, you can import the package from the top Unity Editor toolbar via Assets -> Import Package -> Custom Package
Then from the file explorer select the Nice1 Unity package and import. A window will pop up, hit "OK" to start importing the package into the project.
You can also import the Unity package by dragging and dropping it into the Project tab in the Unity window.