Got it working. The mac desktop 1.0 gave me the same trouble. I had to get access to a Windows machine and set up the old Slingplayer app for Windows (2.0), which successfully updated the firmware and allowed me to use the newer software.
This is really a terrible situation, though. Sling could be doing much better software than this. The ONLY Mac client that works with this Slingbox now is the web player, which sucks in many ways, and even that had to be set up via a Windows machine.
Considering how aggressively Sling soaks its customers for money, they should be supporting their customers a lot better than this.
The places where used Slingboxes are being old are full -- and I do mean full -- of negative reviews slamming the company's official support. If I'd realized that up front I would not have bought this device.
I have a used sb300-xxx. First, the specific hardware setup:
Router: Apple Airport Extreme b/g/n. Wired connection.
Cable Service: Time Warner
Cable Adapter: Motorola Surfboard SB6121
Ports: Just to be on the safe side, I have opened TCP ports 5001-5009, 5101-5109, 5201-5209, 5301-5309, and checked that ports are open.
Cable: I have used known good Cat 5 and Cat 6 cables.
When I power up the Slingbox everything looks fine. All lights come on. Watch and setup both see the box. Airport Utility shows a good connection and internal IP address.
When I go to Directory, it tells me (rightly so) that the firmware needs updating. I click Next, and it proceeds to download. The little progress gauge goes to 25%. (I should add that I can see the network activity during the download. So far, so good.)
Then the connection and network lights go out then come back on a couple of times, and it tells me that it cannot update "due to bad internet connection", which is nonsense. The internet connection is fine.
What gives? I don't understand what the problem is because my internet connection is fine. The problem appears to be with the update software on Slingbox's end.