From Platinum Arts Sandbox Free 3D Game Maker
1. You have to open sandbox
2. select the area which you want to make grass onto. http://i36.tinypic.com/zkhocl.jpg
3. Press F2 and select the grass texture (look at the picture!). http://i33.tinypic.com/29xek50.jpg
4. Type "grass 1" into command line ("grass 0" to disable) http://i38.tinypic.com/22hylf.jpg
And thats how easy it is :)
tutorial by Siimvuss