On the create a listing page, sell similar or revise an item, up the top right corner you will see a link to change to the advanced listing took. Click on that. Ignore the warning and opt to change. If you are revising an active listing, there will be a link to that listing on the next screen. Clicking on that will open the advanced lister and most of the details should carry over. You can then follow DG's advice if it's not showing.

If you're creating a new listing, after you select to change to the advanced lister, the next screen you'll need to key in some keywords so it opens up where you can choose your category.