You don't need to do anything special to use your own currency symbols. The script uses whatever language your site is in by default and should automatically use the correct symbols.

If that doesn't fit your needs or doesn't seem to work, you can always change it in the php.

Find the mod_formula.php file.

Look for:

$math = '"€"+addCommas2(roundNumber('.$f.','.$result->dec.'),'.$result->dec.')';

and change the currency symbol

$math = "'HERE"+addCommas2(roundNumber('.$f.','.$result->dec.'),'.$result->dec.')';