I don't know why this has been omitted in the English version, if it is a miss (they will, hm, probably not admit that) or if they think you don't deserve such rules (that would be strange) or if they have a great masterplan to add it later (even more strange since the Zone Riders was introduces in that ZC) or something completely different. They will hopefully answer. Here is the Swedish rules.
The Talent Zone Rider (my direct quick translation)
You can tame wild bitterbeasts and other animals to be your loyal mount. To tame an animal, you must first capture one. Then you need three successes, with the Manipulate or Dominate skill, in a row to tame the animal. You can roll one roll per day. Failure means that the animal flees or attacks - and that the whole process must be restarted
Besides that you can tame the animal, you are a skilled rider and knows the art to fight mounted. All your movement is doubled in combat - each maneuver spend on movement counts as two. All that Fight you from the ground gets a -2 modification. You can also let your mount fight for you - you use the animals Strength but your skill in Fight. This replaces your action in the round. You can push the skill roll. When it comes to the Bitterbeast you can let them spit acid on your enemies - you use the bitterbeasts Agility but your skill in Shoot.
Rules ride mounts (my direct quick translation)
Anyone can sit on a tame bitterbeast or other tame animal and slowly move through the Zone. When it comes to Zone exploration, a mount counts as a vehicle, all exploration takes half the time.
Conflicts is something else.
Anyone that lacks the talent Zone Rider (see above) must succeed with an Endure roll to stay on the mount. This roll doesn't count as an action or maneuver. A failure means that the beast has thrown you off and you take damage with three basic dice - each Nuke yields one damage.
Anyone who attacks a mounted enemy can choose if to attack the beast or the rider.