Magento Module not loading

here is my config.xml

    <?xml version="1.0" encoding="UTF-8"?>
<config>
    <modules>
        <zee_Checkout>
            <version>1.0</version>
        </zee_Checkout>
    </modules>
    <global>
        <models>
            <zee_Checkout>zee_Checkout_Model</zee_Checkout>
            <checkout>
                <rewrite>
                    <type_onepage>zee_Checkout_Model_Type_Onepage</type_onepage>
                </rewrite>
            </checkout>
        </models>
    </global>
</config>

here is my code/local/zee/Checkout/Model/Type/Onepage.php

class zee_Checkout_Model_Type_Onepage extends Mage_Checkout_Model_Type_Onepage
{

      public function saveOrder()
    {
        $this->validate();
        $isNewCustomer = false;
        switch ($this->getCheckoutMethod()) {
            case self::METHOD_GUEST:
                $this->_prepareGuestQuote();
                break;
            case self::METHOD_REGISTER:
                $this->_prepareNewCustomerQuote();
                $isNewCustomer = true;
                break;
            default:
                $this->_prepareCustomerQuote();
                break;
        }
....
.....
Only this function needs to be Over-rided...so I wrote just this funciton, nothing else in this file except this function

}
}

The Issue is.... this function is not loading... rather saveOrder() from parent / core magento file executes...

Module is enabled and its shows in Magento Backend.. But code doesn't execute

Any Idea??? am I missing something??