A powerful plugin that will allow you to add custom code or CSS to your header, footer, sidebar, content or feed.
Navigate to Plugins within your WordPress Admin Area
Click "Add new" and in the search box enter "Add to All" and select "Keyword" from the dropdown
Find the plugin in the list (usually the first result) and click "Install Now"
Download the plugin
Extract the contents of add-to-all.zip to wp-content/plugins/ folder. You should get a folder called add-to-all.
Activate the Plugin in WP-Admin.
Goto Settings » Add to All to configure