SlingTV is actually a separate service all together; a streaming program through Dish. Nothing to do with Slingbox itself. I'm not sure why this isn't letting you access Slingbox.com; I just attempted to access the page and it just loaded up without a problem. I would make sure your browser is up to date and try to clear the cache and cookies.
As for the Roku player; the SlingPlayer channel doesn't actually stream, just allows you to mirror the stream from another device. You have to have the SlingPlayer on a mobile device of some soft (for example, SlingPlayer for iPhone); once this is on the same network as the Roku and you have the player open, you should see what looks like a little box with a Wi-Fi symbol inside it. When you click this, it will give you the option to stream on the device or on the Roku.
I hope this helps!!