error CS0246: The type or namespace name `BuildingPlacement' could not be found. Are you missing a u

Ok, I can't ask this question on answers so I do it here.
I got this error:
error CS0246: The type or namespace name `BuildingPlacement' could not be found. Are you missing a using directive or an assembly reference?
And I don't know how the f* I should solve this! Please help me thnx.
code:
using System;
using UnityEngine;
using System.Collections;

public class BuildingManager : MonoBehaviour {
public GameObject[] buildings;
private BuildingPlacement buildingPlacement;

// Use this for initialization
void Start () {
buildingPlacement = GetComponent<BuildingPlacement>();
}

// Update is called once per frame
void Update () {
}
void OnGUI () {
for (int i = 0; i <buildings.Length; i ++) {
if(GUI.Button(new Rect(Screen.width/20,Screen.height/15 + Screen.height/12 * i, buildings.name))) {
buildingPlacement.SetItem(buildings);
}
}
}
}

 

That error is saying that Unity can't find a class/script named BuildingPlacement.

 

But there I have a script called BuildingPlacement!

 

Is BuildingPlacement in the same namespace as BuldingManager?

 

nope

 

then at the top of that script you need to add the namespace, you do that by the using keyword, e.g.
using System;
includes the System namespace

 

?
What do I need to Add?

 

I'm new to unity so..

 

if BuildingPlacement is in a different namespace, you need to add using NameSpaceHere; to the top of the script.