I think this is the problem:
Code:

---

text highscoore_txt 
{	
       font = standard_font;
       layer = 31; 
       pos_x = 500;
       pos_y = 275;
       string = highscoore; // <-- WRONG !
}

---

A var is a var and not a string, this are two different data types!
However you can do this:
Code:

---

var highscoore;

string highscoore_str; //BRAND NEW!

text highscoore_txt 
{	
       font = standard_font;
       layer = 31; 
       pos_x = 500;
       pos_y = 275;
       string = highscoore_str;
}

function open_highscoore()
{
  //.. your code up to the file_var_read
  highscoore = file_var_read(scoore_temp);
  file_close(scoore_temp);

  str_for_num(highscoore_str,highscoore); //<-- BRAND NEW!
  highscoore_txt.visible = on;
}

---

What you do is converting the var highscoore to a string highscoore_str

Btw: highscore is written with one "o"