If you have a recent version of Visual Studio 2008 or 2010, it is fairly straight forward. To find the current port assigned to the device, open your Device Manager, in the Control Panel in the System Dialog Window, look under Ports in the device tree. You'll see your USB/serial device and port number if the drivers were installed correctly and the USB device is currently plugged into the system.
If you are not familiar with C++/CLI, the managed C++ required to access the .NET extensions, you may have to find a tutorial, there are significant differences with ISO C++, developing the app in C# might be a shorter learning curve, if allowed.
You should be able to download a free version of Visual Studio, sufficient to develop the app.