The package and version of MvcMailer needed depends on both the the MVC and the .NET version you are using:
ASP.NET MVC3
Install-Package MvcMailer3
ASP.NET MVC4 with .NET 4
Install-Package MvcMailer -Version 4.0
ASP.NET MVC4 with .NET 4.5
Install-Package MvcMailer
This will likely change again when MVC5 and .NET 5 come out.