When you are using the Stacks in Snow Leopard, there is not indication of which highlighting of the elements when you move your mouse over them. If you want to enable highlighting in the Stacks, you can do so with this terminal command:

defaults write com.apple.dock mouse-over-hilite-stack -boolean YES

Once you do this, you need to restart the Dock. There are a variety of ways to do this, but this terminal command is easy enough since you are already in the terminal:

killall Dock

Once the dock restarts, highlighting will be enabled.

If you decide you don't want the highlighting, you can turn this off by running the command again and changing the YES to NO.