The Plat Extras cert doesn't show up online in your account, it is just sent out after every three stays AFTER you have acheived Plat for the year (if this was the first time). I believe they also send you out a Plat Extras cert right away right when you qualify for Plat for the first time.
Could it be possible that the above poster didn't get his cert because he is a non-USA GP member? Either that or his address could be oincorrectly registered? The certs aren't nearly as valuable outside the USA since the RC upgrade benefit is only valid in North American Hyatt properties as of early 2005.