Electronic – Using FreeRTOS in Atmel Studio 6.2

FreeRTOS doesn't care about the environment you are using for development. So yes, you can use MacOS when you are developing with it.

The .exe file you are mentioning is a self-extracting ZIP file (at least that's whats mentioned on the page you are linking to), which is also available as plain ZIP file.

When you are looking at the supported hardware list you find some examples using MPLAB (for PIC18, 24, dsPic and PIC32). These pre-configured examples may make your start easier.

I am trying to create a TickCount.

You'll need a global variable(big enough) to hold the number of seconds(49.7*24*60*60*1000 = 32bit long value OR a ulong) elapsed, increment it inside your isr( it seems you did this part right !).

volatile uint32_t elapsed_ms;

then you'll need function that returns this value.

uint32_t TickCount(void){
 return elapsed_ms;
}

Now we come to part two:
TickCountSys increases once every milli second

If you want to do it with 8MHz Xtal + Timer 0 Overflow, then you cannot achieve precise 1ms ticks. Reason being 8000 clock pulses can not overflow the timer 0. If you use no prescalar then, T0 overflows = 8000/256 = 31.25 times. if you prescale the timer clock with T/8 setting then 1000/256 = 3.90625 close to 4 Overflows.

Assuming that you chose to go ahead with the T/8 pre-scalar, then you'll need to modify the isr body to something like

      static uint8_t temp_count=0;
        temp_count++;
      if(temp_count >= 4){
        temp_count = 0;
        elapsed_ms++;
        //add a method here to roll the 49.7 day completion.
      }

That's it.
From your code it seems you've done the timer and interrupt settings correctly. But your code is quite fuzzy and I failed to locate any function named TickCount or similar.

I'd suggest you to start off with a simple code, like blink a led on timer0 overflow( with a HIGH PRESCALAR value like T/1024). Then gradually add on the code logic. Good Luck.