Like Pacnet said,
You can no longer use the mission editor to make maps with interiors and terrain
However, if you're planing on running it on version 20 or lower, then you can.
But, I can see that you obviously have no experience with making add-ons.
Before you EVER attempt to make an add-on, you need a better understanding of how they work.
You need to understand Torque Script, the gaming engine that Blockland uses.
Without this, you will NEVER be able to design add-ons.
You will also need what's called "Mission Editor".
With this, you'll be able to edit a map from within the game.
You'll have to learn how to use it yourself.
I also don't feel like looking for a download, so you'll also have to find it yourself.