<select name="mySelect" id="mySelect">
    <option value="1" id="option1">1</option>
    <option value="2" id="option2">2</option>
  </select>

var options = $$('select#mySelect option');
var len = options.length;
for (var i = 0; i < len; i++) {
    console.log('Option text = ' + options[i].text);
    console.log('Option value = ' + options[i].value);
}

// replace 1 with index of an item you want to select
options[1].selected = true;

$$('#mySelect option').find(function(ele){return !!ele.selected})

var selectThis = 'option1';
$$('select#mySelectId option').each(function(o) {
  if(o.readAttribute('value') == selectThis) { // note, this compares strings
    o.selected = true;
    throw $break; // remove this if it's a multi-select
  }
});

$('mySelect').setValue(1); // or whatever value you want to select

var myChoice = '2';

$$('select#mySelectId option').each(function(o) {
    o.selected = o.readAttribute('value') == myChoice;
});

var itis = $(mySelectId).select('option[value="' + sValueToSelect + '"]');
if ( itis && itis.length > 0 )
    itis[0].selected = true;

var selectThis = 'option1';
$$('select#mySelect option').each(function(o){
      if(o.id==selectThis){o.selected = true;$break;}
});

$('mySelect').value = 2; // 2 being the value you want selected

document.observe('dom:loaded', function() {
   $('mySelect').observe('change', function(event) {
    ...
   });
});