<form name="myForm">
<input type="checkbox" onclick="selectWhenChecked(0)">Burger
<input type="checkbox" onclick="selectWhenChecked(1)">Dog
<input type="checkbox" onclick="selectWhenChecked(2)">Nachos
<select name="mySelect">
<option>Fries
<option>Kraut
<option>Salsa
</select>
</form>
<script>
function selectWhenChecked(myNewChoice){
document.myForm.mySelect.options[myNewChoice].selected = true;
}
</script>