Which version of VS.NET and MVC are you using.

If using VS.NET 2008, check your web.config whether it has this entry?

<httpModules>
			<add name="UrlRoutingModule" type="System.Web.Routing.UrlRoutingModule, System.Web.Routing, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
		</httpModules>

The UrlRoutingModule evaluates a request and checks if it matches a route that is stored in the RouteTable. If the route matches, it overrides the default handler (IHttpHandler) for the request so that the MVC framework handles the request.

From ASP.NET MVC in Action book.