spiros on "[Plugin: WordPress SEO by Yoast] Changing og:locale value"

WordPress SEO version: 1.3.4.4

I am running a Greek blog, however in see the source:

<meta property='og:locale' content='en_US'/>

I want to change that to Greek. I tried this:
http://wordpress.org/support/topic/plugin-wordpress-seo-by-yoast-how-do-you-set-the-oglocale-value?replies=2

But gave me this error:

Fatal error: Call to undefined function add_filter() in ...wp-includes/functions.php on line 3852

http://wordpress.org/extend/plugins/wordpress-seo/