Have you even tried the code you posted? It works perfectly as supposed..?
Code:
#include <acknex.h>

function writeXML()
{
	var file = file_open_write("out_info.xml");
	STRING* str = "";
	
	str_cat(str,"<out_info>\n\t");
	str_cat(str,"</out_info>");
	
	file_str_write(file,str);
	
	file_close(file);
}

void main()
{
	writeXML();	
}

Always learn from history, to be sure you make the same mistakes again...