The plug-in is installing, you just need to allow it to run through Safari's preferences.
Click through these steps with Safari running:
Safari (top of the screen near the Apple logo) > Preferences > Security (tab).
In here click the "Plug-In Settings" button next to Internet Plug-Ins, then "WebSlingPlayer Plug-In". Set this to Always Allow and click Done.
Next, click Advanced (tab) and remove the check form the box next to "Stop plug-ins to save power".
This should allow the player to work without looping back to the Installation page.
Hope this helps!
Sling Media Moderation Team