I had the same problem with my Mac. We found out that because we downloaded the Beta Yosemite OS Slingbox didn't have a plugin for it. Since Yosemite came out with their "real" Yosemite recently we installed it, got the plugin and now can watch again. Unfortunately, now when I start my Slingbox it turns on the TV too.
I'm having this same issue, works fine from all my mobile devices running the slingbox app, but not from my MAC. I haven't gone to Yosemite yet, so still running on version 10.9.5. Have had it working in the past, but can't seem to get it to work again.
Sling needs better support or a better method of connecting to slingbox. why not the ability to connect direct to the slingbox via my a web browser? Seems ridiculous that I have to connect to a site outside of my network in order to connect back into my own network???