MoveObject.cs

using UnityEngine;
using UnityEngine.Networking;
using System.Collections;

public class MoveObject : NetworkBehaviour {

    public GameObject ourObjectToMove; //Public so we can see it in the inspector, but making this private won't hurt you.

    RaycastHit hit; //Will contain the data from the ray we shoot in Update()

    void Start () {
        if (!isLocalPlayer)
            this.enabled = false; //This is just a temporary fix to make sure that our script isn't active on this object if we aren't the local player (so that when we press space it doesn't send a command from every player!)
                                  //You should make a seperate script to do this in the future however.
	}

	void Update () {

        //I don't know how you want to determine what object gets affected. Here's one solution, but this can be applied to pretty much anything.

        if(Input.GetMouseButtonDown(0)) //We click with the left mouse button
        {
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition); //Shoot a ray out from our mouse
            if(Physics.Raycast(ray, out hit)) //If our ray hits something
            {
                ourObjectToMove = hit.transform.gameObject;
                Debug.Log("We have clicked on " + hit.transform.name + " and made it our selected object!");
            }
        }

        if (Input.GetKeyDown(KeyCode.Space) && ourObjectToMove != null) //We have an object to move and we have pressed the space bar.
        {
            print("Space pressed");
            CmdMoveObject(10, Vector3.up, ourObjectToMove.GetComponent<NetworkIdentity>().netId); //We need to get the netID of whatever object we are trying to move.
        }
    }

    [Command]
    void CmdMoveObject(float power, Vector3 direction, NetworkInstanceId netid)
    {
        RpcMoveObject(power, direction, netid); //Commands are only called on the server. We want this to be replicated onto all clients as well, so we call a ClientRPC.
    }

    [ClientRpc]
    void RpcMoveObject(float power, Vector3 direction, NetworkInstanceId netid) //It gets passed the same arguments.
    {
        GameObject ourThingToMove = ClientScene.FindLocalObject(netid); //This will have each client get a reference to the thing we want to move by searching for its netID.
        ourThingToMove.GetComponent<Rigidbody>().AddForce(power * direction, ForceMode.Impulse); //Now we can apply the force!
    }
}