You must be at least level 82.
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Alliance
Look for the Hero's Call Board in an Alliance capital city, find the quest 
[82] Hero's Call: Deepholm! and follow the quest chain.
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Horde
Look for the Warchief's Command Board in a Horde capital city, find the quest 
[82] Warchief's Command: Deepholm! and follow the quest chain.
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Returning to Deepholm
After completing the initial quest line, you can teleport back to Deepholm from the portal hubs in Orgrimmar or Stormwind.
copied from http://wow.answers.wikia.com/wiki/How_do_I_get_to_Deepholm
