How add meta tag to a specific page in WordPress

How about creating a page template? Just copy the usual page.php into a new file mymeta-page.php or something and add the meta tag. Then for the page(s) you want to have that meta tag, just assign the template to them when you create/edit the page(s).

See http://codex.wordpress.org/Pages#Creating_Your_Own_Page_Templates for details.

Or you can use the slug:

http://codex.wordpress.org/Pages#Templates_by_page-ID_or_page-Slug

This latter would probably be my preferred option.

HTH

PAE