You have to work for a dealer or Mimaki. They won't train independent techs. That being said, if you have experience fixing machines in the field, you can find a dealer who might contract service calls out to you, do some jobs and build a relationship, then ask them to send you to training. It's free to the dealer but you would have to get yourself there and pay for a hotel. It's a win win because the dealer can use you for warranty work after that and you get the training. That all depends on the dealer though.
As a certified Mimaki tech I can tell you, they basically teach you how to install the machines properly. There isn't a ton of troubleshooting info that's gone over so if you don't have natural troubleshooting skills, the training won't help that much. If you're good at troubleshooting and fixing machines already, the certification really only allows you to do warranty work which doesn't pay well.