The plugin “Coupon In Url” is the light weight extension for Magento. It is the simple extension that allows administrator of the Magento store create URLs with coupon code.
Using “Coupon In Url” extension administrator can include the link with coupon code and gives the opportunity to customers use the coupon without typing it in the field on the checkout page.
If the customer click on the such link
http://yourmagentostore.com/?coupon=SALES50 on the checkout page and on the cart page automatically if it is availabe for current customer’s quote.2) Then You should disable the cache 3) Unzip the archive with the extension 4) Be carefull when uploading the content of the archive by ftp in root folder of your Magento, You should MERGE the content.
1) First of all You should disable Compilation here ([ Admin Area ]: System -> Tools -> Compilation )