Try clearing your cache and cookies, or rebooting the computer. You could also try uninstalling and reinstalling the SlingPlayer plug-in for your browser. Below are a few articles that should help with this issue:
- Slingbox.com - Troubleshooting Watch on Slingbox.com installation or usage
- Slingbox.com - Installing/removing browser plug-in for Watch on Slingbox.com
Hope this helps out!