I have been stepping through the source code for Orchard CMS to learn how they have tackled Multi-tenancy.
OrchardProject
Can someone point out the files or
You need to install the multi-tennancy module from the orchard gallery, enable it, and configure it in settings.
The code you want to look at is, the code that is in the multi-tennant module subfolder.
However, knowing how Orchard does multi-tenancy won't much help you. Orchard has a very complex routing and extension system, with tons of layers and dynamic dispatch points. You can't simply drop in Orchard's internal system into a simple MVC app without, basically, recreating all of Orchard's infrastructure.
Orchard is not a good example of how to write a stand-alone MVC app, because it has so many requirements for flexibility, and its system is so heavly based on the dynamic claysharp stuff.
Unless there is a reason you need to use the Orchard method specifically, you'd be better suited looking at solutions like this http://weblogs.asp.net/zowens/archive/tags/Multi-tenancy/default.aspx