Don't forget that they also want you to declare some schemas to the html tag - specifically the open graph one via xmlns:og="http://opengraphprotocol.org/schema/. OG is based on RDFa which adds the additional attributes to the meta tags.
Once you're done with that, be sure to add your fb:like - <fb:like href="http://developers.facebook.com/" width="450" height="80"/> - the example is pretty straightforward. Don't forget to add xmlns:fb="http://www.facebook.com/2008/fbml" otherwise you will be sad
Found this answer on SO regarding validation of the meta tag. Doing what that answer describes should give let you pass it through any w3c validator.
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