Maps are distributed as zip files. To install them, simply save the zip file to your Blockland/Add-Ons folder. Do not extract the zip file.
To locate your Add-Ons folder:
* Right click your Blockland Shortcut
* Click "Properties"
* Click the "Find Target" button ("Open File Location" in Vista)
* This will open your Blockland folder
* Open the folder labled "Add-Ons"
* This is where you put new Maps
Once a map is installed, it will show up in the map list in the "Start a Game" dialog.