Error compiling using GSM SendSMS code

I got this error after compiling

C:\Users\ChrisAnthony\Downloads\arduino-1.5.7\libraries\GSM\src\GSM3CircularBuffer.cpp: In member function 'void GSM3CircularBuffer::debugBuffer()':
C:\Users\ChrisAnthony\Downloads\arduino-1.5.7\libraries\GSM\src\GSM3CircularBuffer.cpp:269:2: error: 'Serial' was not declared in this scope
Serial.println();
^
C:\Users\ChrisAnthony\Downloads\arduino-1.5.7\libraries\GSM\src\GSM3CircularBuffer.cpp: In static member function 'static void GSM3CircularBuffer::printCharDebug(uint8_t)':
C:\Users\ChrisAnthony\Downloads\arduino-1.5.7\libraries\GSM\src\GSM3CircularBuffer.cpp:282:3: error: 'Serial' was not declared in this scope
Serial.print((char)c);
^
C:\Users\ChrisAnthony\Downloads\arduino-1.5.7\libraries\GSM\src\GSM3CircularBuffer.cpp:285:3: error: 'Serial' was not declared in this scope
Serial.print('%');
^

Pls help