Internal Compiler Error :/

I'm trying to load the SmartFox DLL, but it's being lame.

 Internal compiler error. See the console log for more information. output was:
 Unhandled Exception: Mono.CSharp.InternalErrorException: Assets/_Scripts/Networking/NetworkingClient.cs(6,14): NetworkingClient ---> Mono.CSharp.InternalErrorException: Assets/_Scripts/Networking/NetworkingClient.cs(8,18): NetworkingClient.s ---> System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies.

Not sure, but I feel like I'm using .Net 4.0 because of the "Version=4.0.0.0" line and I can't figure out how to change it.

Error is on line: public class NetworkingClient : MonoBehaviour {

Line that causes error: SmartFox s;

Full Script to deduct error: using UnityEngine; using System.Collections; using Sfs2X; using Sfs2X.Core;

 public class NetworkingClient : MonoBehaviour {
 
     SmartFox s;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }