You must learn a special skill to tame Cloud Serpents, which is different to the skill required to tame other Lesser Dragonkin. Pandaren hunters gain the skill automatically, but hunters of other races must learn it from the tome How to School Your Serpent, which may purchased from San Redscale at The Arboretum in The Jade Forest. You require Exalted standing with The Order of the Cloud Serpent to read this tome, but the learned skill is account-wide. You can even purchase and read this tome on a non-hunter character.
The Foot of Lei Shen, The Fall of Shan Bu (Isle of Thunder) (Scenario)
Retains its original name after taming. This version of Nalak appears in the very last stage of the The Fall of Shan Bu, the final scenario in the (one-time) Isle of Thunder intro quest chain. He will activate after you've defeated the troll and mogu NPCs around Shan Bu. He will rapidly fly out of range, so to tame him you should stand to the left of Shan Bu (when facing him from the entrance), on a large disc with a Mogu face, roughly where the mouth is. Start to tame the second he activates.
Level scaling: Most NPCs will scale with the Hunter's level, within the constraints of their level range. Hunters can tame regular NPCs up to 2 levels higher than them, but can only tame elite NPCs of the hunter's level or below.