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

https://docs.unity3d.com/560/Documentation/Manual/AssetPackages.html

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

 CrossPlatformInputManager.GetAxis("Horizontal");