You don't have any element with id="nanoIpod", yet to try to address it using getElementById('nanoIpod'). This will definitely fail. So it's time to debug :-)
1. Add an id attribute to the select (let's say "select1")
2. document.getElementById('select1').value will give you the currently selected value of the select inside your onchange handler
3. Depending on the value you can show/hide the depending div's
If you need more info, feel free to ask. Please add your updated code then.