I believe the tile should be adhered to the smooth side.

If you look carefully on the edges along the long side you will see a indent much like you would find on wall board. This is only visible on one side and not the other. This is the smooth side of the board. This indent allows you to tape with fiberglass tape and thin set compound to give you a solid surface in which to place the tile. So smooth side out.

The rough side is the back or bottom...with 1/4 " hardie there is often scoring lines on the front...1/2" just has the printing on the front.