To get a merit you have to leave an especially outstanding and helpful comment (well written with good grammar and capitalization). If the moderators or administrators think that it is worthy, they will reward it with a merit.
On each userpage, there is an area that numbers how many merits you have earned. Merits are earned by writing constuctive, thoughtful comments in the Games section. We reward those who take the time to think out a good comment , as it contributes a lot to the website and sets a good example for others. The post does not have to be a book in length, but should be interesting enough to grab our attention. For each merit you recieve, you will earn 25 AP and your post will stand out. Only moderators and administrators give out merits. You can help us choose who gets the merits by hitting the "up-vote" arrow next to comments on the game pages.
That is an exact quote from the Faq: http://armorgames.com/page/help
Can there be a way to see what post you got a merit for? That would really help a lot. Also, as a tip, if you leave a comment for a game, try to make it a good review covering all points, whether they be good or bad. Also be sure not to repat too much of what has already been said unless you say that you are agreeing with someone.