Assuming you're using https://github.com/devi/kohana-activerecord, the simplest answer is to load the class manually.
require_once('/kohana-activerecord/classes/arm.php')
in your code, and do that until you have all of your dependencies loaded. I'm not entirely sure how the Kohana-Auth library is meant to load properly, and it has no real useful documentation on its github page. But I'll assume just manually loading the classes will be fine as it hasn't been updated for a couple years.