Basically all it does is add the option to change one terrain to either.. grass - sand - jungle - snow - or stone. Most terrains can be changed to the other.. but it costs 1 material to change so it isn't completely free..
To change a terrain to grass it uses 1 mud.
To change a terrain to jungle it uses 1 jungle mushroom.
To change a terrain to snow it uses 1 snowpile.
To change a terrain to sand it uses 1 sand.
To change a terrain to stone it uses 1 stone.
Other wise the untill/tills for normal terrains as they are in the default game are unchanged.. (ie If you want to turn a stone to grass so you can plant a wheat there you need to turn the stone to grass.. then till the grass).
You can turn this mod on/off so your right click menu isn't so long when you don't really need it on.
Picture:

Download:
http://townsmods.net/projects/view/41
Or
http://www.mediafire.com/download.php?m51pq2cak3xupv6
To install:
Just extract the folder in the mod folder in your .towns folder... Then when you turn on the game enable the mod for it to work.
If you want a "Free" version it is simple to edit this mod.. by just going in the actions.xml and deleting the bits that make them use items.. but I think having to pay something is better...
This does not let you rebuild terrain.. or fill holes - that is currently not possible due to how terrain works.. it only lets you change 1 terrain in to some other terrain.
Any issues (maybe some since I expanded it a bit to include most useful stuff.. and didn't fully check it..) let me know... As far as I know it should be fine.... haha.
Otherwise... If possible.. Enjoy..
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also if any want to translate these to Spanish .. I can edit them in:
Use mud to change to grass from
Use jungle mushroom to change to jungle from
Use snowpile to change to snow from
Use sand to change to sand from
Use stone to change to stone from
Remember they say in game (like the picture)
Use stone to change to stone from - terrain your right click-
So whatever sounds most right in that situation..