How to access unused pins on an atmega1280?

You don't need to worry about those lines unless you plan to use PJ2 as a pin change interrupt. However if you do, I have modified them for you:

#define digitalPinToPCICR(p)    ( (((p) >= 10) && ((p) <= 13)) || \
                                  (((p) >= 50) && ((p) <= 53)) || \
                                  (((p) >= 62) && ((p) <= 69)) || \
								  ((p) == 70) ? (&PCICR) : ((uint8_t *)0) )

#define digitalPinToPCICRbit(p) ( (((p) >= 10) && ((p) <= 13)) || (((p) >= 50) && ((p) <= 53)) ? 0 : \
                                ( (((p) >= 62) && ((p) <= 69)) ? 2 : \
                                ( ((p) == 70) ? 1 : 0 ) ) )

#define digitalPinToPCMSK(p)    ( (((p) >= 10) && ((p) <= 13)) || (((p) >= 50) && ((p) <= 53)) ? (&PCMSK0) : \
                                ( (((p) >= 62) && ((p) <= 69)) ? (&PCMSK2) : \
                                ( ((p) == 70) ? (&PCMSK1) : ((uint8_t *)0) ) ) )

#define digitalPinToPCMSKbit(p) ( (((p) >= 10) && ((p) <= 13)) ? ((p) - 6) : \
                                ( (((p) >= 50) && ((p) <= 53)) ? (53 - (p)) : \
                                ( (((p) >= 62) && ((p) <= 69)) ? ((p) - 62) : \
                                ( ((p) == 70) ? 3 : 0 ) ) ) )

In order to get a new pin to work, you need to do a several steps:
(1) at the end of this:

const uint8_t PROGMEM digital_pin_to_port_PGM[]

add this:

	PJ	, // PJ 2 ** 70 ** D70

This specifies which port the pin is on (in this case J)

(2) at the end of this:

const uint8_t PROGMEM digital_pin_to_bit_mask_PGM[]

add this:

	_BV( 2 )	, // PJ 2 ** 70 ** D70

This specifies a bit mask for this pin in its port (In this case it is the bitmask for pin 2)

(3) at the end of this:

const uint8_t PROGMEM digital_pin_to_timer_PGM[]

add this:

	NOT_ON_TIMER	, // PJ 2 ** 70 ** D70

This specifies that it is not a PWM output (which it is not)

(4) at the to, change this:

#define NUM_DIGITAL_PINS            70

to this:

#define NUM_DIGITAL_PINS            71

This tells the IDE there are now 71 useable pins not 70.

Then restart the IDE and try to run the code. It should work.