To use the cross platform input manager you need to make sure you have the Standard Assets Package included in your Unity project. To import it, use these instructions

Ensure that CrossPlatformInput is selected in both the “Editor” and “Standard Assets” folders.

Open up the Project Setting > Inputs to define all of the controls

To refer to the CPI from your class file you must include it at the top with

 using UnityStandardAssets.CrossPlatformInput;

Then you can reference it directly