Question

not sure what is wrong with this jquery code: http://jsfiddle.net/aCABM/

I want to to grey out the dropdown menu when ‘yes’ is selected. When no is selected it can be selected if the user so wishes.

Answer 1

Select the elements by radio button name there is no need to select them individually by there ids and use prop instead of attr since you are manipulating its disabled property. Try this.

$(function(){
    $("input[name='discount']").click(function() {
        $("#discountselection").prop("disabled", this.value == 'Yes');
    });
});

Demo

In your fiddle there were few issues.

1. Missed to include jQuery library
2. You cannot just begin your markup with td, it should be enclosed with table and tr element.
3. In the Js window you just have to put your js, no need to have script tags.