Can't call raycast on plane to get mouse position? Beginner Question

I'm trying to run the sample code from the API here: http://docs.unity3d.com/ScriptReference/Plane.Raycast.html

But it doesn't run. Why can't I call .Raycast on the groundPlane?

 using UnityEngine;
 using System.Collections;
 
 public class ExampleClass : MonoBehaviour {
     public Plane groundPlane;
     public Transform markerObject;
     void Update() {
         if (Input.GetMouseButtonDown(0)) {
             Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
             float rayDistance;
             if (groundPlane.Raycast(ray, out rayDistance))
                 markerObject.position = ray.GetPoint(rayDistance);
             
         }
     }
 }

Yields the following error: Assets/Scripts/Player/old/ExampleClass.cs(11,41): error CS1061: Type Plane' does not contain a definition for Raycast' and no extension method Raycast' of type Plane' could be found (are you missing a using directive or an assembly reference?)

Some online tutorial code also yields a similar set of errors? For starters, when initializing the plane, I can't pass the two parameters, when the API says I can?

 Ray ray = viewCamera.ScreenPointToRay (Input.mousePosition);
         Plane groundPlane = new Plane (Vector3.up, Vector3.zero);
         float rayDistance;
 
         if (groundPlane.Raycast(ray,out rayDistance)) {
             Vector3 point = ray.GetPoint(rayDistance);
             Debug.DrawLine(ray.origin,point,Color.red);
             //Debug.DrawRay(ray.origin,ray.direction * 100,Color.red);
             controller.LookAt(point);
         }