Sorry to hear you are having issues with your SlingPlayer. I have a couple ideas that might help. To troubleshoot your SlingPlayer connection issue it will be helpful delete your player preferences. This article How to uninstall SlingPlayer for Mac explains which files to remove, and note, you can only do this at home (at your Slingbox location/network). Once you dumps these files your Slingbox ID or password will no longer be stored anywhere else on your Mac.
If you are using Lion, your library will be hidden. You'll need to use the Go > Go to folder option to get to the prefs, as referenced in How to find the Internet Plug-ins folder in Mac OSX 10.7 (Lion) article.
The video settings are part of the Audio/Video setup in Setup Assistant, so using Watch on Slingbox.com, you should be able to go in and adjust adjust the brightness, contrast, hue and saturation settings on the Slingbox. The look of Watch on Slingbox.com should be the same as any other SlingPlayer client, such as the desktop player.