If you have having troubles with procuring this mount, try this!
1) Be sure to win 3 games of Hearthstone in PLAY or ARENA mode first! Your 3 wins can be from Casual or Ranked play. The introductory missions do not count. You'll know it's your third win because you'll get the "Mount Up!" quest reward notification in Hearthstone!
2) Log into World of Warcraft and check your mail. No Hearthsteed to be seen? Go to the next step!
3) Log out of World of Warcraft and log in to Hearthstone one more time.
4) Log out of Hearthstone and log back in to World of Warcraft. Check your mail - your Hearthsteed should await!