You Need Modern Warfare 2

Firstly:

Download one of the trial games from the following list;

Metal Slug 3
Metal Slug XX
Samurai Showdown II
The King of Fighters '98: Ultimate Match
Garou: Mark of the Wolves

Secondly:

1)Boot up Modern Warfare 2.
2)Go into a Private lobby. Do not start the game
3)Go to Quick Launch and load up one of the selected arcade games.

Now you should be able to play the trial game as a Full game.
You can get achievements just like you would in the full game, but they will not come up on your games list.
You have to do this everytime you boot up the arcade game.

This method may also work for other games, so try some others out and hit me up if they work
First Post from Greenz.