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");