I\'ve just done a fresh install and was able to access the default rails page at localhost:3000, but when I installed the activeadmin gem I had a problem when accessing /admin/
This is a known issue due to the jquery-rails dependency dropping jQuery-UI support. The workaround for the moment appears to be to force the jquery-rails gem to load using version 2.3.0:
gem 'jquery-rails', '~> 2.3.0'
A related problem due to this: Debug jQueryUI Versions in Rails.