if you use the new launcher, and if you want to play hearthstone for the new mount, it's probably a good idea to try the new launcher, before they enforce it.
ie.
Battle.net
just open up the swirl at the top left corner, the "&&" symbol where it says "&&" "Games" "News"
go to settings, game settings, tick the box for "launch 32 bit client (instead of 64 bit)"
that's it.
in game, you'll have to adjust the DirectX9/11 setting, but it should attach now.
If you use the original launcher, you'll need to rename the file called wow-64.exe in the wow folder to not.wow.exe.bak or some other alteration.
as you may be able to tell, the new launcher is a lot easier to set this.