Firstly: <script language="javascript">
The language attribute does *nothing* - it is a meaningless waste of space.
If you want to identify a script as JS, use type="text/javascript" instead.
Next: Use jQuery.
It will make your existing JavaScript a billion times easier to work
with, and make what you're trying to do now so much easier you'll
wonder how you coped without it.
As for making sure a user has entered a quantity... that existing code
doesn't tell me anything about how your form is laid out, so there's
no way to be sure what you need to do.
Supply the HTML for the form, and I can have a go at giving a solution.
