observer_etc.xml

<?xml version="1.0"?>
<config>
  <modules>
     <mycompanyname_helloworld>
     <version>0.1.0 </version>
     </mycompanyname_helloworld>
  </modules>
  <frontend>
    <routers>
      <helloworld>
        <use>standard</use>
        <args>
          <module>MyCompanyName_HelloWorld</module>
          <frontName>helloworld</frontName>
        </args>
      </helloworld>
    </routers>
  </frontend>
  <global>
    <models>
        <helloworld>
             <class>MyCompanyName_HelloWorld_Model</class>
        </helloworld>
    </models>
    <events>
        <checkout_cart_add_product_complete>
          <observers>
            <mycompanyname_helloworld_observer>
              <type>singleton</type>
              <class>MyCompanyName_HelloWorld_Model_Observer</class>
              <method>addProductToCartAfter</method>
            </mycompanyname_helloworld_observer>
          </observers>
        </checkout_cart_add_product_complete>     
      </events>
  </global>
</config>