No, I'm joining thru the Internet.
This is your problem. You cannot join your own dedicated server through the list of Internet servers. At the bottom of the server list, click the button labeled Query LAN when your dedicated server is running. It should refresh the list with your server being the only one on there. Highlight, click Join. Bada-bing.
If your server does not show up in the LAN list, close your server, close Blockland, re-open Blockland, then re-open your dedicated server, and do what I said to join it.