The target framework for the project needs to be 4.0 or higher. With Visual Studio 2013, unless you changed the framework when creating the project, it'll be 4.5. Also, be sure to reference System.Numerics.dll in your project as well, since it's not a default reference.
See my original post -- it doesn't likeImports System.Numerics
Imports only opens up the namespace for you so that you don't need a fully qualified name every time. You still need to reference the assemby, otherwise the namespace simply doesn't exist in the project.
And as you can see from my second post .NET 4.5 is installed, which meets the requiremenet of 4.0 or higher.
Installed and targeted are two completely different things. I have 4.5 installed, but if I only target 2.0, I can't use anything in the 4.5 framework. Look at your project properties.