Didn't you ask this in one of the multiple threads you created over the past few weeks?
Anyhow, to do it, simply divide the matrix width by the amount of xtiles ( this will give you the width of each tile ) and the matrix height by the amount of ztiles ( this will give you the length of each tile ).
Then, get the object's X position, and divide it by the width of the tiles, and do the same for the Z position, and voila, you should have the tile number...
eg:
`earlier in the program
Make Matrix 1,1000,1000,50,50 : `ie, Make Matrix 1,width,depth,tileswide,tilesdeep
tilewidth = 1000/50 : `ie, tilewidth = width/tileswide
tiledepth = 1000/50 : `ie, tiledepth = depth/tilesdeep
`and in the loop
tilex = Int(Object Position X(obj)/tilewidth)
tilez = Int(Object Position Z(obj)/tiledepth)
`Voila, the object is positioned at tile tilex,tilez
Hope I Helped...

Team EOD :: Programmer/Logical Engineer/All-Round Nice Guy