I finally figured out what was going wrong.
All the router and network settings were correct.
My slingbox was apparently set to "WAN Access? No."
I discovered this by looking at the Information tab in the "Slingbox Directory -> Edit" Window.
I'm not sure why it was set to disallow WAN access; I reran the configuration setup and explicitly allowed this capability.
It works now (confirmed this by having a friend install the Mac Slingplayer, connect in from their house, and verify)!