It seems that you are having some issues with the SlingPlayer for Android Tablets since the button to change among the video quality options is gray.
In this case, I recommend you to uninstall and reinstall the SlingPlayer for Android Tablets application and test the missing feature. If you do not know how to, just go to Settings and select Applications. Go to Manage Applications and select the SlingPlayer for Android Tablets. Then, press the Uninstall option. Finally, go to the Android Market Place and reinstall the SlingPlayer application attached to your Google Account.