How do you get to the second floor in the mine on harvest moon ds?

If you mean how you get to lower levels, then when you walk around the mine, you will fall in a hole and it will transport you to a lower floor of the mine. However, it is hidden, so you don't know where the spot is. Also, when you "fall" in a hole of a mine, it hurts you a little. Taking away a little of your stamina. You can FIND the hole by using your Hoe and tilling squares until it is found. There is ALWAYS at LEAST ONE hole and ONE pair of stairs. The stairs will not affect your health if you use them.

If you mean unlocking all the mines, it sounds as if you unlocked the first one. In order to unlock Another mine, you must travel to the last level of the most recent mine and defeat the dark animals. Once you have done that, you may go home and travel BACK to the Mine Site sometime after 10pm and Carter will come in the mine and ask you to knock down a wall with your hammer. This will unlock the next mine and everyone after.