Serial was not declared in the scope

Using Arduino Installation & Troubleshooting
#1

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

#2 
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

#3

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.

#4