If you want to add some code to all pages (like google analytics tracking code) go to System and Settings in Dashboard, and there to Tracking Codes in SEO & STATISTICS section. There you can paste code, which will be in every page on your site - you can even select if it will be in header or footer section.
If you want to add something to specific page - hover Edit, click properties and in Custon Attributes tab click Header Extra Content - this will give you text area, where you can put part of code that will be in header of that page.
Last option - you can go to your theme in themes (or packeges - whether your theme is) directory via ftp and add some code to header.php, or for instance some page type php document.