First off, you can't really put it into 'onload', as it is an event you want invoked whenever the user clicks on the elements, not something you want performed once. You could, however, attach an event handler of a framework at the onload if you wanted it handled that way, but really there is no need for something simple as this unless you had more elaborate things planned.
Maybe I should have explained the code more, I probably assumed you had a little more basic knowledge of JavaScript, sorry, I tend to do that alot.
CODE
document.getElementByID('idOfElement')
As the name of the function states, it grabs the element in the document which has the id of what is passed, so here it would be whatever has 'id="idOfElement"' as an attribute. It returns this, so in the example code I gave you, I accessed the CSS style element of the DOMObject returned. From there, the style object can be used as you would anything CSS, so I altered the 'visibility' to change from visible to hidden, depending on which radio button you clicked.
Ok, with that aside, I've taken a look at your page. The main issue I saw was you are missing an ending tag for your table row that the div is in, but that shouldn't break the functionality. I've tested the code myself and it works fine with one error in the error console stating that 'default' is not a value for the CSS display property. Looking into it, you should make it 'display:inline', as that is the default. Should work just fine, I've tested it myself with no problems, though I do see it not working on your source site.