just put a block wich you didnt use before, a -P block and not a -@ block. then when you want to change this block into something it will ask you i dont remember what but say ok ^^ then the whole list of blocks will appear and you will choose StaduimGrass but warning, if a block is mixed with the grass or it's ust on it the grass will disappear, just like the water does. if that happens just put nother grass block in the same location of the first one
i put StadiumGrassClip,dunno for other grass
stadiumgrassclip is the attachement between some blocks for example the road blocks but not the standard one, you know they have 2 things at the end when u put them alone but when u put another road block next to it, this thing will attach the 2 blocks. so thats not a good block for the grass