Start dedicated.bat in your Blockland folder, then open Blockland as usual, and join your own server. The name will be the same as the last server you hosted. Once joined, you can used RTB Prefs Manager or something to change settings.
NOTE: The first time you host a dedicated server with RTB, it will bring up some text and ask you to enter a command to set up the server. You might want to wait for that to come up before you start Blockland to join)
If you can't find your server, you can also press the Query LAN button at the bottom (it should show just your dedicated server) I think.
Hope that wasn't too confusing.