How do you instantiate an object that is not included in your C# project?

Question

Note: All sample code is greatly simplified.

I have a DLL defined as:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Web;

namespace RIV.Module
{
    public interface IModule
    {
        StringWriter ProcessRequest(HttpContext context);
        string Decrypt(string interactive);
        string ExecutePlayerAction(object ParamObjectFromFlash);
        void LogEvent(object LoggingObjectFromFlash);
    }
}

Now, outside of my solution, other developers can define concrete classes and drop them into the BIN folder of my app. Maybe something like:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using RIV.Module;

namespace RIV.Module.Greeting
{
    public class Module : IModule
    {
        public System.IO.StringWriter ProcessRequest(System.Web.HttpContext context)
        {
            //...
        }
        public string Decrypt(string interactive)
        {
            //...
        }
        public string ExecutePlayerAction(object ParamObjectFromFlash)
        {
            //...
        }
        public void LogEvent(object LoggingObjectFromFlash)
        {
            //...
        }
    }
}

Now, in my app I would need to know that a new Module was available (I am guessing via web.config or something along those lines) and then be able to call it based off of some trigger in the database Campaign table (which maps to the module to use for that specific campaign).

I am trying to instantiate it this way:

var type = typeof(RIV.Module.Greeting.Module);
var obj = (RIV.Module.Greeting.Module)Activator.CreateInstance(type);

However, the compiler belches because a reference was never set to RIV.Module.Greeting.dll!

What am I doing wrong?