Because pin assignments are implementation-specific, this SDK provides this enumeration with only the constant GPIO_NONE defined. When the SDK is ported to a specific hardware platform, it is expected that those who do the port will use this enumeration as the basis of an enumerated type for that platform that defines the GPIO pin assignments. Doing so will enable the platform-specific enumeration to be cast to the type Cpu.Pin. In this way, all SDK methods maintain type safety while still enabling full portability.
To use the correct pin definitions, you probably need to include a using statement in your code. For example, an application that uses the sample Freescale implementation would require the following statement in its .cs files:
using Microsoft.SPOT.Hardware.FreescaleMXSDemo;