There are a few things that you could do. I would start with clearing the cache and cookies on your computer. If that does not help, you may want to try and uninstall the SlingPlayer plug-in then go back to the website to reinstall it. Below are a few links that will help with the suggested steps:
- 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!