Can this be modified to have FALLING or RISING interupts, or do I simply check inside the ISR the pins for HIGH or LOW?
No, this is strictly for pin CHANGE interrupts.
Yes, you will have to implement your own system of detecting rising vs falling.
Mind you, remember to keep your interrupt service routine short, as when in a heavy ISR block, you run the risk of missing another interrupt. Depending on the device you have hooked up on this pin, consider debouncing.
Finally, turn off interrupts when you're inside an ISR, as you shouldn't allow an interrupt while you're already servicing one.
#define atomic(off) PCICR = 0x07 //sets the pin change interrupt control register to B00000111
#define atomic(on) PCICR = 0x00 //sets the pin change interrupt control register to B00000000
Now in your ISR you may write something like:
ISR ( PCINT1_vect)
atomic(on); //masks all interrupts. ie. sets the interrupt mask
atomic(off); //resets the interrupt control registers to enable interrupts
Be warned, depending on whether or not you are actually using all 3 pin change interrupt capabilities, you might need to modify the hex values in the atomic(on) define statement above. Please see section 12.2.4 in the atmel data sheet for more details.
Some good reading, although doesn't have anything on pin change interrupts, but I believe that you will need to include the headers that they include in their code snippet. http://gonium.net/md/2006/12/20/handling-external-interrupts-with-arduino/
Good luck. Let me know if you have issues, as I haven't tested the above code.