PayPal Express Checkout Button Not Showing in Magento 1.9.0.1 – Troubleshooting Guide

Have you tried changing address_override to 0 rather than removing it?

I'd shy away from editing that file directly as it'll be kicked out if you upgrade magento. Better to create a copy of that file in /app/code/local/Mage/Paypal/Model/Api/Nvp.php and modify that.

(thanks to https://magento.stackexchange.com/a/26102 for that)

Check that you have the following xml in your paypal.xml layout:

<checkout_cart_index>
    <reference name="checkout.cart.top_methods">
        <block type="paypal/express_shortcut" name="checkout.cart.methods.paypal_express.top" before="checkout.cart.methods.onepage.top" template="paypal/express/shortcut.phtml">
            <action method="setIsQuoteAllowed"><value>1</value></action>
            <action method="setShowOrPosition"><value>after</value></action>
        </block>
    </reference>

    <reference name="checkout.cart.methods">
        <block type="paypal/express_shortcut" name="checkout.cart.methods.paypal_express.bottom" before="checkout.cart.methods.onepage.bottom" template="paypal/express/shortcut.phtml">
            <action method="setIsQuoteAllowed"><value>1</value></action>
            <action method="setShowOrPosition"><value>after</value></action>
        </block>
    </reference>

    <update handle="SHORTCUT_popup" />
</checkout_cart_index>

Also make sure you clear Magento cache so that the changes can reflect.

If that doesn't work, enable Magento logging and go to the cart page. If it does not display, check exception.log for any Paypal gateway issues.