Thank you for the post on the Slingbox answer forum. This is a very good question you have. It sounds as though something on the router where the Slingbox is located is causing the Slingbox to be blocked by the MAC and not the PC. I will post a few links I would like you to look over to see if we can get that corrected for your MAC.
Hope this helps,
Sling Moderation Team
Thank you for your email.
My Slingbox is located at my daughters home. Your technical support remotely re-set my Slingbox, after which I did exhaustive tests from my home (10 miles away) and all these tests were very successful. The tests were done over an 8 day period. Please see original message.
Therefore I think it would be safe to conclude that the port forwarding function of the router (at my daughters home) has been set correctly.
Are you still of the opinion that port forwarding is the problem?