Head to the area marked F. The arrow indicates that you can only go one way here. Once you drop down, you can't go back the same way.
Once you've dropped down, continue along the path to the door in the southeast marked G.
If you haven't done so before, you must examine the door 6 times; the description changes each time. Eventually you receive a message saying "you see something silver glittering around the indentation" in the door.
At level 63 there are easy prey monsters near the end that are just low enough not to aggro.
The most popular way to raise your fame for this quest is to complete the quest Mihgo's Amigo with 212 necklaces, or the Shady Business quest with ~104 ores. (12/2016 edit: 36 necklaces were enough. May be less, 24 necklaces gave fame 3.)
A way to check your Norg fame without travelling to Norg is to check the price of Rice Balls with Ghebi Damomohe in Neptune's Spire. If the price of the Rice Balls are 150g or lower, you should have sufficient reputation for this quest (prices for Rice Balls may vary a little). (However, the Reputation page claims 155 or lower).