Sounds like you've not defined the global object Serial. In the Arduino environment this is defined in HardwareSerial.h / HardwareSerial.cpp
Have you got this file in your library?
If so are you including HardwareSerial.h or WProgram.h?
Remember that eclipse isn't like the Arduino IDE. It doesn't do anything for you. You HAVE to include all the headers yourself, declare all functions and include all libraries for the linker.
Iain