1

using UnityEngine;
using System.Collections;
using UnityStandardAssets.Characters.FirstPerson;

public class InputEnabler : MonoBehaviour {
static FirstPersonController FPSChar;

// Use this for initialization
void Start () 
{
	FPSChar = GameObject.FindObjectOfType<FirstPersonController>();
	Disable();
}
public static void Disable() // disabling cursor and enabling movement
{
	Cursor.visible = false;
	Cursor.lockState = CursorLockMode.Locked;
	FPSChar.enabled = true;
}

public static void Enable() // enabling cursor and movement
{
	Cursor.visible = true;
	Cursor.lockState = CursorLockMode.None;
	FPSChar.enabled = false;
}

}

using UnityEngine;
using System.Collections;

public class Inventory : MonoBehaviour {

public GameObject inventory;

// Use this for initialization
void Start () {

}

// Update is called once per frame
void Update ()
{
	if(inventory.activeSelf)
	{
		if(Input.GetKeyDown(KeyCode.I))
			//Disable Inventory
			inventory.SetActive(false);
		InputEnabler.Disable();

	}
	else
	{
		if(Input.GetKeyDown(KeyCode.I))
					{
						//Emable Inventory
						inventory.SetActive(true);
			InputEnabler.Enable();
					}
	}
}

}

2

You have neither declared InputEnabler or gotten a reference to it in your Inventory inventory class.

In Inventory.cs you need to do one of two things, this is the easiest

public InputEnabler inputEnabler;
void Start ()
{
    if (inputEnabler == null)
    {
        Debug.LogError("There is no InputEnabler component set in the inspector. Please set one.");
    }
}

otherwise you can do this

private InputEnabler inputEnabler;
void Start ()
{
    inputEnabler = GetComponent<InputEnabler>();
    if (inputEnabler == null)
    {
        Debug.LogError("There is no InputEnabler component attached. Please attach one.");
    }
}

Now you need to make these changes

InputEnabler.Disable();

and this

InputEnabler.Enable();

to this

inputEnabler.Disable();

this and

inputEnabler.Enable();