I don't understand how it wouldn't know, the o-scope reads what is there, maybe you connected improperly?
Also your code doesn't make much sense. The code that the tutorial wants you to follow has you doing shiftOut() commands, in which you will need to define your clock pin and your data pin along with the already defined latch pin.
If you are using a mega and you did make an 8x8x8 led cube, you aren't exactly following that tutorial at all. Although if you are referring to the wiring diagrams in that tutorial, then you could easily use the o-scope to check to see if the pins are going high. Check up on how to use an O-scope real quick, it only takes a few minutes.
now as for your current code, i'll help you out.
const int latchPin = 77;
With this definition, the arduino doesn't know you have 595's there. to the arduino and every other code in the world, this is just another interger definition.
void setup() {
pinMode(latchPin, OUTPUT);
}
This is just another definition of an output, nothing odd there. Just not definitive of a 595 or an led cube
digitalWrite(latchPin,HIGH);
delay(1000);
So this is pretty normal, you are turning whatever is connected to latchPin for 1000ms, nothing wrong here.
//digitalWrite(latchPin,LOW);
//delay(1000);
Heres a problem, with both of these lines commented out, you'll be looping over and over and over again trying to turn an already high pin, high again. So with these commented out, there is nothing turning it back low, and thus no need for any delay methods. If you want to keep these two commented out, i would suggest changing the code to this:
void loop() {
digitalWrite(latchPin, HIGH);
}
That way it will always be high, with less bits of code.
Did that help?