As this is an official plugin, it is documented as part of the main MountainBerryFields code.
If you have it installed (gem install mountain_berry_fields-magic_comments), then it will be automatically loaded by MountainBerryFields, no further steps are necessary.
If you are using Bundler, make sure it is in your Gemfile. If using Rubygems, set it as a development dependency
(example).