I've had guest access work, and this is what was required..
When you set the Slingbox up, it will ask you for all the various passwords, both the admin and guest password. It will also give you a unique identifier for guest access.
What has to happen is the person you want to allow view access to your Slingbox has to create an account at Slingbox and then log in to that account. When asked for a slingbox to connect to, they then punch in the unique identifier and guest password. It sounds like you're trying to do guest from either the Slingplayer app or an account logged in...or from your own network. From what I can tell, guest viewing only works over the internet, and local access is Admin by default.