tress is a pointer, int_mas[0] is not a pointer. try this:
Code:
#define jauns(struktura) sys_malloc(sizeof(struktura))
#define jauni(struktura,daudzums) sys_malloc(daudzums*sizeof(struktura))

typedef struct 
{
   int i;
}Str;

function main()
{
   Str* otrs;
	Str* tress;
   Str* int_mas = jauni(Str, 10);
   
   int lok_i;
   
   
   for(lok_i = 0; lok_i<10; lok_i++)
   {
      int_mas[lok_i].i = lok_i;
   }
   otrs = int_mas;
	tress = &int_mas[0];// <===== works now
	
	printf("tress.i: %.3f",(double)tress.i);
}