Dark Iron can only be smelted at the Dark Iron Forge in Blackrock Depts, which is located deep into the instance. To gain the smelt Dark Iron Ore ability, you will also need to do a quest given by the dwarf Gloom’rel in the ghost dwarf room. The quest requires 230 engineering. Bring Gloom’Rel 20 Gold Bars, 10 Truesilver Bars and 2 Star Rubies to learn your new smelting ability. Making one Dark Iron Bar requires 8 ores.
Note: Blizzard has removed the need for Dark Iron Ore in Burning Crusade. It is not necessary for any schematics over 300 skill, and is way too expensive for advancing your Engineering above 300.