I have a slpit level home. I pured a concrete patio and I want to build a deck leading DOWN to the patio. How do I handle the deck posts?

Install them so they are 48 inches below the ground level and outside of the patio area.

I have a funny feeling you are going to tell us that the concrete patio covers the entire area where the posts are needed. If so, drill and bolt in a beam bracket to hold the 4x4 post. http://www.agindustrialsupply.com/(S(de3xgp45c3szkw55vmbfam3w))/productdetails.aspx?sku=10182808&source=GoogleBase

Yes putting the metal post brackets down, is a must, but if you rest the weight of a deck on 3-4 posts, and only have a 4" slab you will have some issues later.

The point load on the posts could push the concrete down over time and even cause cracks at those points.

If possible pour some kind of footings, or post pads and place the posts on them.

Then there are a whole bunch of issues on putting the deck together correctly, so it looks good over 5 years. There are a lot of "bad" decks out there.