Making maps is simple.

Fist of all, copy the template map and rename the copy <level>.map (where <level> is the level number).
When adding a new map, the level number should be 1 higher than the last existing map, else the game won't recognise it.

you can then alter the information in the map. Take a look at one of the included maps to get the idea.
The key is as follows:

0 - Empty
1 - Wall
2 - Earth
3 - Diamond
4 - Rock
5 - Exit
6 - Start point
7 - Baddie (only 10 allowed!)

For the inexperienced: To view a map, either load it into Notepad, or right click -> open with and select notepad.
