Short Answer:
You just need to move your dataaccess layer into a WCF service, expose it and then call it from your MVC application.
Long Answer:
1) I recommend going through Scott Guthrie’s Nerd Dinner walkthrough to get a really good understanding of MVC basics.
2) The following blog goes through the process of writing a Layered ASP.NET MVC Applicaiton with a WCF Service (Parts III & IV specifically concentrate on WCF & MVC interaction):
- Part I (General Concepts)
- Part II (Building a Repository)
- Part III (Building The Service)
- Part IV (Using The Service)
I haven’t read the linked arrticles in detail (just skimmed them) but they seem to give a decent overview of the process.