How do I tame a zombie horse?

"Adult horses, donkeys, and mules can be tamed, but not zombie horses or skeleton horses. With an empty hand mount the horse repeatedly; when it no longer bucks the player and shows hearts, it is tamed. It is necessary to tame a horse in order to breed it, give it equipment, or ride it for any length of time. Taming depends on the horse's "temper". Horses begin with a temper of 0 out of 100. When a player is riding the horse, a random number 0–99 is chosen. The horse becomes tame if this number is less than the temper, otherwise the temper is increased by 5 and the player is bucked off. Temper can also be increased by feeding the horse."

http://minecraft.gamepedia.com/Horse

You should however be able to summon a tamed zombie horse by using commands:

/summon EntityHorse ~ ~ ~ {Type:3, Tame:1}

by typing this in chat and sending it, you should have summoned a zombie horse.