I can't move my box !!! with rigidbody constraints

using UnityEngine;
using System.Collections;

public class sc_Player : MonoBehaviour {
float xspeed=0f;
float yspeed=0f;
float sp=8;
float grav = -1;
float gravlim= -12;
float jumppow = 18;

	public Rigidbody Rbod;

	// Use this for initialization
	void Start () {
		Rbod = (Rigidbody)gameObject.AddComponent ("Rigidbody");
		Rbod.useGravity = false;
		transform.rigidbody.constraints = RigidbodyConstraints.FreezePositionZ;

	// turn's gravity off
	//transform.rigidbody.contraints = rigidbody.contraints
	void Update () 
		xspeed = 0f;

		if(Input.GetKey ("right")) xspeed= sp;
		if(Input.GetKey("left")) xspeed = -sp;
if (yspeed <= gravlim)yspeed = gravlim;
if(Input.GetKeyDown("x")) yspeed = jumppow;

rigidbody.velocity = new Vector3(xspeed,yspeed,0);
transform.rotation = Quaternion.AngleAxis (0, Vector3.forward);

Here is my question, I want to use a cubed based collision detection, but for some reason when I tried to add a gravity variable, it made it impossible to move my cube ? I just don’t really know what’s constraining the right and left movement, I was thinking of doing some sort of raycast to stop the downwards momentum, but i don’t know is that’s really the problem,or a good solution.The odd thing is that I’m able to move right and left when I use a capsule instead of a cube.

Eh I know you probably already know this, but transform.rigidbody.constraints don’t work anymore because that is no longer part of unity
You have to do gameObject.GetComponent