In Silvermoon City you need to go the Sunfury Spire, pictured in the map below (it is near at the top of the map, near the middle). The Inner Sanctum has a teleport which will take you to the Undercity. 

![Sunfury Spire is at the top in the middle][1]

Directly outside the Undercity are zeppelin towers, pictured below.  **The tower on the left in the picture will take you to Orgrimmar.** There are Goblins at the front of each landing pad who will tell you which city this zeppelin is for, and where it is on its trip (intended to give you an idea of how long you'll be waiting). For example: *The zeppelin for Orgrimmar has just left Orgrimmar*. 

![Tower on the left has the zeppelin to Orgrimmar][2]


  [1]: https://i.sstatic.net/TUPOp.jpg
  [2]: https://i.sstatic.net/zfA6e.jpg