You don't put them anywhere. Moderators and Administrators give them out at their own discretion of a user leaves a detailed, useful comment on a game. Merits are sometimes/often given out for contest winners as well.
"Never say never" I say and with that being said... =P
I think you should never stop with writing reviews (yes, this too is a way to earn a merit) just simply because you don't think you will get a merit for them?!
The first and foremost reason for writing reviews on games is... giving your review on a game! 'Duh' you might think right now, I know, but still... just keep writing them! And when they get around to looking at them again, who knows?! You might get a whole bunch of them at once! Nice thought huh?