Disclaimer: I've been asked to state that the use of a boggle solver can result in a ban from the developers. Please use at your own risk
This achievement requires you use all 16 tiles in one game, not necessarily one word.
If you're having trouble getting this on your own, try using this boggle solver and make sure you use every tile at least once.http://wineverygame.com/boggle/helper.pl
Edit: If you're not able to unlock this, keep in mind this is in beta and only open to US accounts. If your Live account is in another country you will likely not be able to unlock these achievements at this time.