Well, blow me down. I thought I understood the Arduino pin mapping, but obviously not. For some reason I thought that these graphics that are common around the web would have some relation to the pin names used in the IDE:
So I'm looking at PK6, which is pin A14 and I see that TX3 is PJ1 and also pin 14. Now, for PJ1 is that just "14" or can it also be "D14" or "TX3" when used in a statement? Further, I see that PJ2 through PJ7 don't have an Arduino pin assignment, but I'm betting that they can be manipulated using direct port commands such as "PORTB |= _BV(7)" as in KRUPSKI'S example.
BTW, thanks for the pin assignment graphic.
But neither of your responses answer the question of why "A8" works in the compiler, but "PK0" does not, yet it doesn't create a compiler error. So "PK0" is obviously defined in the IDE--does that mean it is only appropriate in a certain class of statements, eg, direct port manipulation, but not in things like digitalWrite()?