The Zandalari Trolls and Kul Tiran humans were added as Allied Races in the 8.1 patch of World of Warcraft. Despite this they were not readily available at launch of the path, and we needed to wait a bit longer before we could unlock them as a playable Allied Race.
The earliest possible unlock date for the Zandalari Trolls and Kul Tiran humans are when the new raid Battle for Dazal'alor opens, since you will need to kill the endboss of this raid to complete the quest required to begin the unlocking scenario.
This means that the earliest date when it's possible to obtain the Zandalari Trolls and Kul Tiran humans are on January 22 in the US, and January 23 in EU. At this date both normal and heroic version of the raid will open. If you plan on killing the final boss with Looking For Raid you will need to wait until February 26/27 when the final wing opens for LFR.