In order to install Happy Birthday module, please, follow this steps:
  1. Extract module archive to the directory with your Magento
  2. Login to admin area, clear all Magento cache(System->Cache Managment) and logout from admin area
  3. Login to admin area and go to module configuration page(System->Config->Birthday greetings) and setup module
  4. Create Shopping cart rule for customers which have a birthday(Promotions->Shopping Cart Price Ruls)
  5. Edit default greeting message template accordingly with your needs
  6. Add http://[YOUR_STORE_URL]/birthday/birthday/ URL to cron tab (or you can run it manually once in a day)