Just edit app\design\frontend\default\default\template\page\html\head.phtml and add the custom meta tags in there. Or maybe the path is different if you set up a new template for your website (maybe app\design\frontend\yourtemplate\default\template\page\html\head.phtml)
or
Please go to System > Configuration > Design > Html Head and edit “Miscellaneous scripts” field. Unfortunately this works only for static information / meta-tags.
See more about SEO on Magento:
http://www.screenpages.com/about/articles/seo-in-magento
