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 () {
	
	}
}

Okay I don’t know SmartFox but lets go through the basics for troubleshooting.

1 - Do you have the player set on .net or .net subset? It looks like smartfox is trying to access System.IO with the .net subset does not allow.

2 - If this is a precompiled .lib does it need to be in a plugin directory? And does it require Pro?

i see the answer of Ethan in this thread : InternalCompilerError when I import the new Facebook 5.1 SDK into my existing project - Stack Overflow