custom editors?

Unity Engine
1

Hello i have been trying to make a custom editor but first i wanted to play with custom editor example from unity website but i did every thing they say and i put it in editor folder and it still wont work.
It gives me a error if i use AT letter(@).Where is custom editor.

@CustomEditor (LookAtPoint)

class LookAtPointEditor extends Editor {
    function OnInspectorGUI () {
        target.lookAtPoint = EditorGUILayout.Vector3Field ("Look At Point", target.lookAtPoint);
        if (GUI.changed)
            EditorUtility.SetDirty (target);
    }

    function OnSceneGUI () {
        target.lookAtPoint = Handles.PositionHandle (target.lookAtPoint, Quaternion.identity);
        if (GUI.changed)
            EditorUtility.SetDirty (target);
    }
}
2

What’s the error message?