IIS problem - To grant this application the required permission please contact your system administrator

I need assistance. I have a web application that I have installed on an IIS 6.0 server. But, I get this specific error:
I need to know where on the IIS server to go to turn off security to allow castle.monorail applications to run on the IIS server.

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  

Stack Trace:


[SecurityException: That assembly does not allow partially trusted callers.]
   Castle.MonoRail.Framework.EngineContextModule.CreateAndStartContainer(HttpApplication context) +0
   Castle.MonoRail.Framework.EngineContextModule.Init(HttpApplication context) +96
   System.Web.HttpApplication.InitModulesCommon() +65
   System.Web.HttpApplication.InitModules() +43
   System.Web.HttpApplication.InitInternal(HttpContext context, HttpApplicationState state, MethodInfo[] handlers) +729
   System.Web.HttpApplicationFactory.GetNormalApplicationInstance(HttpContext context) +298
   System.Web.HttpApplicationFactory.GetApplicationInstance(HttpContext context) +107
   System.Web.HttpRuntime.ProcessRequestInternal(HttpWorkerRequest wr) +289