First, I think the issue isn't stemming from the Slingbox itself, but from the Slingplayer Web Plugin software you're using to try to configure the Slingbox. First, I would make sure the Slingplayer Web Plugin is downloaded and up to date. Here is an article with some information on installing the plugin (the process is the same for updating the plugin):
Once the plugin install seems to have gone through and is working properly, head back over to setup.slingbox.com, and see if it works any better for you.
If for some reason you can't get that plugin to work properly, an alternative is using the older Slingplayer software to get the Slingbox configured. You can find the download appropriate for your computer in this article here:
It'll be either Slingplayer 2.0 for Windows, or Slingplayer 1.0 for Mac.
Hope this helps!