Serial was not declared in the scope

How to get rid from the following error.Iam using ardupoter with atmega 2560.

Building for ArduPilot Mega 2.x
Excluding arduino core from include paths
MultiSerialMega.ino:22:21: error: Arduino.h: No such file or directory
MultiSerialMega.ino: In function 'void setup()':
MultiSerialMega:24: error: 'Serial' was not declared in this scope
MultiSerialMega:25: error: 'Serial1' was not declared in this scope
MultiSerialMega.ino: In function 'void loop()':
MultiSerialMega:30: error: 'Serial1' was not declared in this scope
MultiSerialMega:32: error: 'Serial' was not declared in this scope
MultiSerialMega:36: error: 'Serial' was not declared in this scope
MultiSerialMega:38: error: 'Serial1' was not declared in this scope

MultiSerialMega.ino:22:21: error: Arduino.h: No such file or directory

Serial being missing is the least of your worries.

Clue: Excluding arduino core from include paths

You need Arduino 1.0 or higher to compile that sketch. I recommend the latest (currently Arduino 1.0.6) if your operating system supports it.