Allied Races: Dark Iron Dwarf

If you're exalted with 7th Legion and have completed the Ready For War achievement, you need to go to Stormwind - specifically the embassy - to begin a quest chain that will take you to Blackrock Mountain (scenario) among others.

As bhob and datgrl say you have to complete the following

1 - Be Exalted with the 7th Legion
2 - Complete Ready For War Achievement
3 - Head to Stormwind Embassy
4 - Complete Blackrock Mountain Scenario
5 - Complete THE MOTHERLODE dungeon on any difficulty level
6 - Head back to Stormwind embassy to hand in the quest

And then complete these other scenarios

7 - You will get sent to The Molten Core for another quest
8 - Ride the tank, spamming button 1, and into the entrance where Ragnaros normally would be.
9 - You will get removed from the tank so fight your way to the end to face Gezzroc The Keeper. Beat him and you will have a mole machine ride back to SW
10 - Hand the quest in and then you will be transported to the Firelands
11 - Follow it straight over the bridge killing everything in your way. Then confront High-Justice Grimstone and kill him.
12 - Stand in the fire at the edge of the lake while you fill up the core. When you reach 100% a mole machine appears behind you.Hand quest in in the embassy.
13 - You are now sent back to Blackrock Mountain to repair the Black Anvil. Click on the orb on top of the anvil to repair it.
14 - Return to Stormwind (via mole machine that is behind you) to speak to Anduin.
15 - Congrats Dark Iron Dwarves are now unlocked.

I hope this helps.

In addition to the quest chain and scenario, you also need to run The MOTHERLODE. The Wowhead guides are rather misleading on this aspect of the Allied races, stating only that you need Ready for War and Exalted Rep with 7th Legion.