Here this observer will display an additional message whenever a product is added to the cart.

Here the name space is PHPCodez

1) Change the directory to the root of magento installation

2) vi app/etc/modules/PHPCodez_Checkout.xml

3) Paste here the below given code

<?xml version=”1.0″?>

<config>

       <modules>

               <PHPCodez_Checkout>

                       <active>true</active>

                       <codePool>local</codePool>

               </PHPCodez_Checkout>

       </modules>

</config>

4) mkdir app/code/local/ (Run this command if local directory is not created )

5) mkdir app/code/local/PHPCodez

6) mkdir app/code/local/PHPCodez/Checkout

7) mkdir app/code/local/PHPCodez/Checkout/etc

8) mkdir app/code/local/PHPCodez/Checkout/Model

9) vi app/code/local/PHPCodez/Checkout/etc/config.xml

10) Paste here the below given code

<?xml version=”1.0″?>

<config>

<modules>

<PHPCodez_Checkout>

<version>0.1.0</version>

</PHPCodez_Checkout>

</modules>

<global>

<events>

<checkout_cart_product_add_after>

<observers>

<PHPCodez_Checkout_Model_Observer>

<type>singleton</type>

<class>PHPCodez_Checkout_Model_Observer</class>

<method>CustomerEdit</method>

</PHPCodez_Checkout_Model_Observer>

</observers>

</checkout_cart_product_add_after>

</events>

</global>

</config>

11) vi app/code/local/PHPCodez/Checkout/Model/Observer.php

12)  Paste here the below given code

<?php

       class PHPCodez_Checkout_Model_Observer {

               public function CustomerEdit($observer) {

                       echo Mage::getSingleton(‘checkout/session’)->addSuccess(“Inside Obserevr”);

               }

       }

?>

13) Clear the cache and check it by adding the product to cart .

Download source file

Hope it would be helpful