I just rewrote you code and it works without a problem:
 Code:
#include <acknex.h>
#include <default.c>

var a = 1;
var b = 2;
var c = 0;

PANEL* pDisplay =
{
	digits(35,10,"a = %0.f",*,1,a);
	digits(35,19,"b = %0.f",*,1,b);
	digits(35,28,"c = %0.f",*,1,c);
	flags = VISIBLE;
}

function main()
{
	video_mode = 6;
	screen_color.blue = 150;
	while(1)
	{
		c = a+b;
		wait(1);
	}
}


Other than that function is C-Script, use the datatype of the return value instead which doesn´t exist in your function main and thus "function" should be "void" in the case. But that has nothing to do with your problem...