Your application credentials need to have embedded access make see this example work locally. The example uses Ruby on Rails as a backend framework but the Shopify Embedded API is JavaScript based, so it is backend language/framework agnostic. Use this as a reference.
Clone the repo from git:
git clone https://github.com/Shopify/embedded-app-example.git
cd embedded-app-example
To add your application credentials:
cp config/shopify_app.yml.example config/shopify_app.yml
Open the config/shopify_app.yml
and add your credentials.
Install the gems:
bundle install
Run the server:
bundle exec rails server
To install the application on your dev-shop go to:
http://localhost:3000/login?shop=<yourdevshop-url.myshopify.com>
You will be prompted to install the application and will be redirected to the embedded Shopify environment once installed.
Copyright (c) 2013 Shopify. Released under the MIT-LICENSE.