What version of browser are you using? I understand that the sling player browser app is not (yet) compatible with Firefox version 7. Nor with any version of Google Chrome.
CORRECTIOIN EDIT BY CHAKKARINEN
Many thanks to phrelin, who found the solution to the Firefox 7 update issue. He writes:
"After I installed FF7 in its own directory and clicked on my Sling bookmark and the "watch" link, it went through the updated add-on install routine. It works fine."