The ActiveAdmin gem was working for me in development, but not in production. The reason, I found, was because not all of the assets used by ActiveAdmin were getting precompiled by Heroku. To solve the issue, I used the code by Ryan Long’s answer on Stack Overflow (in production.rb).
# Precompile additional assets. Defaults to [application.js, application.css, non-JS/CSS] config.assets.precompile += ['active_admin.css', 'active_admin/print.css', 'active_admin.js']