Any way to have job sub categories display?

Hi

How would I get job sub-categories to show up on the 'post-a-job' form?

Here's a link...

http://www.greenwichmums.com/jobs-experts/jobs/post-a-job/

So, in our case I'd like to be able to pick from a main category of 'Childcare' and then 'Au Pair' from a secondary drop down menu.

Thanks for your help!

James.