New Software Serial Library isn't getting compiled

I wanted to download the software serial library and found that the old one has been replaced by new one. The New Software serial library isn't getting compiled.
This is the error message:
In file included from NewSoftSerialExample.pde:1:0:
C:\Program Files (x86)\Arduino\libraries\NewSoftSerial/NewSoftSerial.h:33:2: error: #error NewSoftSerial has been moved into the Arduino core as of version 1.0. Use SoftwareSerial instead.
#error NewSoftSerial has been moved into the Arduino core as of version 1.0. Use SoftwareSerial instead.
^
In file included from NewSoftSerialExample.pde:1:0:
C:\Program Files (x86)\Arduino\libraries\NewSoftSerial/NewSoftSerial.h:99:16: error: conflicting return type specified for 'virtual void NewSoftSerial::write(uint8_t)'
virtual void write(uint8_t byte);
^
In file included from C:\Program Files (x86)\Arduino\hardware\arduino\sam\cores\arduino/Stream.h:26:0,
from C:\Program Files (x86)\Arduino\libraries\NewSoftSerial/NewSoftSerial.h:40,
from NewSoftSerialExample.pde:1:
C:\Program Files (x86)\Arduino\hardware\arduino\sam\cores\arduino/Print.h:48:20: error: overriding 'virtual size_t Print::write(uint8_t)'
virtual size_t write(uint8_t) = 0;
^
Error compiling.

What should i do?
Where can I download the old software serial library?

It doesn't work on the Due because it's dependent on the interrupts on the AVR core.

Why can't you use the Due's 5 hardware serial ports?