1.) Does your Astrobox contain a WiFi dongle and it's own hotspot? If so, you'll need to connect to this (or be on the same WiFi network as the Astrobox) before going to
http://astrobox-1234.local. This is the easiest method.
2.) If your Astrobox is connected via Ethernet, you'll have to figure out it's IP address somehow. Did you go through the setup process? If so let, you should already know the IP address. If not, try going into your router settings to see connected devices and you should see your Astrobox and it's IP address.
We're you a recent Kickstarter backer? If so, we released desktop and mobile apps for backers (in beta) that makes this process even easier as it will automatically find any Astroboxes that are connected to your network.
Also, please skim through this guide to make sure that your setup process is correct.
Let me know if that helps