From your question, I understand that you are running Windows 8 UI (formerly known as Metro), and not the Windows 8 desktop.
In this case, you need the Windows Mobile Mobile Broadband API and SMS API for Windows 8.
Here is the documentation for Windows.Networking.NetworkOperators, which includes mobile broadband.
Here is the documentation for Windows.Networking.Connectivity, which includes network adapters.
Here's a guide to using Win 8 Mobile Broadband.
Here is the documentation for Windows.Devices.SMS.
Here is a tutorial on using the SMS-API of Windows 8.
Note that they are different from the APIs that will be used for the Windows 8 desktop application. These APIs are described here.
EDIT: The Windows 8 UI is the green part with the fragments that you see when you first start Windows 8. The Windows 8 desktop is the traditional Windows interface that can be accessed by clicking on the desktop tile. Applications in the user interface must use various Windows API applications from applications on the desktop.
From another question that you posted, I saw that you do not have the right drivers for Windows 8, and that also affects what you can do.
To use any Windows Mobile Broadband API, you must be able to see your device as a Windows network adapter. To check this, connect the device and open the Windows device manager and check if your device is displayed in the Network adapters section. If so, then you can use the Windows API. If not, then you cannot use these APIs.
There is another possibility of using the device as a GSM modem, and these are AT commands, as suggested in the answer to your other question.
To be able to use AT commands, your phone must set the modem port, and you can see it in the Windows Device Manager. You can access this modem port from the Windows 8 desktop application. BUT, I'm pretty sure that you cannot access the modem port and use AT commands from the Windows 8 interface (but check this).
There are many examples of the rest of the Internet how to open a modem port and send and receive AT commands from a .NET application.
If you get only yellow triangles in the Windows device manager or if your device is not visible at all, you should get the correct drivers or get another modem that is recognized in Windows 8.