When you create an Isometric map, when you choose the width and height of the tiles, set the width to double that of the height.
Link to example : http://i47.tinypic.com/fykb9w.png
(its a 1920 x 1080 picture, hence why i'm linking it... its MASSIVE on a forum haha).
EDIT: Just a heads up... if you plan to use this... be prepared to mess around with positioning and sizing for ages until it works right ¬_¬ it's probably more trouble than it's worth haha.
I was messing around with, and ended up having the maps tile size at 48 Width and 25 Height. The tiles i drew were within that size. I ended up with 5 tiles to draw 1 individual 3d looking Box.http://i48.tinypic.com/33y0wh2.png
EDIT2: Something else to take note of.. if you show the grid, the tiles don't line up with it. I don't know if it's a bug in Tiled or if its the sizes I chose, but it places them properly anyway... I don't know if that will cause any problems when it's rendered in slick though.
Finally...good luck to anyone planning on using this O_O .. I don't think I have the patience to use it haha.