You cannot reference assemblies compiled for .NET Microframework (.NETMF) from a project written for Windows Forms and vice versa.
Netduino and .NETMF in general does not support Windows Forms. The purpose of the USB connection between your Netduino and your PC is primarily to allow programming and debugging from your PC running Visual Studio.
This does not mean the Netduino can't can't interface with a GUI application running on your PC but you have to establish some kind of communication link between the two parties. Usually, this is done via either Ethernet or a serial link.
Also, you can write a GUI to run on your Netduino, but you'll need a display (LCDs are commonly used) and probably a library for it. There are quite a few options for this, have a look at this post: