Unfortunately, it isn't possible to jump to a specific page. The numbers at the bottom move you through the document in percentage jumps (1 is 10%, 2 is 20%, and so on)
If you hold down the forward or back buttons it will jump 10 pages at a time. Using both of these you can get fairly close to where you want to be...
Jump to page has been on everyone's wishlist since the Reader was released though... we already have the number keys.