If you want to know how to add coupon codes and discounts in magento store, please follow the instruction below:

First set up your rule information in Magento. Log in to your admin and go to Promotions>Shopping Cart Price Rules:

Select Add New Rule and enter a rule name and description.Then select Yes for Enabled.

After completing the form above, click save and continue to edit. Then select Actions from the sidebar menu.

The screen will resemble the one below. Fill out the form with the correct discount percentage.

Then click save to create your new rule. Customers will now be able to enter your promo code at checkout to receive the discount.

To create more advanced conditions such as buy 1 get 1 free or applying discounting to certain products, please refer to this link.

http://www.magentocommerce.com/knowledge-base/entry/what-are-shoppi…

Hope this helps in knowing about How to add coupon codes and discounts in magento