Could be a firewall. Try disabling it, or lower the security setting temporarily. If it still doesn't work outside of the home then turn it back on.
As for the port forwarding, you would only have to do that if your slingbox was connected with wifi. Yet you mentioned that it's connected with ethernet to your Tivo? It doesn't work like that. The slingbox would have to be connected directly to your router if you want it hardwired.