Do you have a liquid template handler? Otherwise Rails won't know what you want to do with .liquid files. See this blog post: http://royvandermeij.com/blog/2011/09/21/create-a-liquid-handler-for-rails-3-dot-1/
For your second question: not using a theme for app/views/admin/*
you should make sure you have an AdminController that does not prepend_view_path
.