The advanced adjustments in the Highlights & Shadows tool were combined into the "Mid Contrast" slider in Aperture 3.3 and later. If you have any images in your library that were processed in a version of Aperture before 3.3, there will be an Upgrade button in the Highlights & Shadows tool in the upper right, and the controls you asked about under the Advanced section. Clicking the Upgrade button will re-render the photo using the new version of Highlights & Shadows, and the Advanced section will be replaced with the new Mid Contrast slider. With the new version from 3.3 you probably don't need the Advanced slider, but if you want to use the older version you can download it from this page:
In addition to heff_sf's advice you may want to have a look at the thread that Kirby Krieger started, when this change to the Highlight&Shadows adjustment first has been intorduced:
Many thanks for clarifying this for me. I've downloaded the fix and added it to the effects tab. I wonder why you can't add it to the add adjustment tab as this is where it was originally. I find the new mid contrast slider a bit heavy handed and it seems to undo the effect of the shadows slider. Anyways, at least I now know what's happening. Thanks again.