Make sure you start your dedicated server properly. The following sequence works perfectly for me every time, see if it works for you:
1. Start Blockland.
2. Open your Blockland folder, wherever it may lie.
3. Double-click (run) dedicated.bat
4. Wait for the dedicated console window to say "Authentication... SUCCESS" or something to that effect.
You are now running a dedicated server. To join it, go back to your Blockland window, click Join A Game, then at the bottom of the server list, look for the button that Sonicfan mentioned, "Query LAN". Click that, and the server list should refresh with your dedicated server being the only server on that list. You know what to do from there.