Your translation isn't entirely off, but your problem is not one of type and would have happened in the original javascript.

Here's a literal translation of the first script:

using UnityEngine;

public class Shoot : MonoBehaviour {
    public Transform bullet;
    public float shootForce;

    void Update() {
        if (Input.GetButtonDown("Fire1")) {
            Transform clone = Instantiate(bullet, transform.position, 
                                          transform.rotation) as Transform;
            clone.rigidbody.AddForce(transform.forward * shootForce);
        }
    }
}

For help with translation, this is a useful link.

Your problem is in one of these possible locations:

• Instantiate(bullet... - is bullet set to something?
• clone.rigidbody... - does bullet have a Rigidbody?

You can add some checks to avoid this problem altogether:

using UnityEngine;

public class Shoot : MonoBehaviour {
    public Transform bullet;
    public float shootForce = 1000.0f;

    void Update() {
        if (Input.GetButtonDown("Fire1") && bullet && bullet.rigidbody) {
            Transform clone = Instantiate(bullet, transform.position, 
                                          transform.rotation) as Transform;
            clone.rigidbody.AddForce(transform.forward * shootForce);
        }
    }
}