Go to -
Halo: CE - Custom Game
Game mode
Options
File Options (if the option doesn't show up, change a random setting, then click it)
Save As
Type in .fortune
Hit enter
You'll get a prompt menu showing up giving you a "fortune".
The achievement should now unlock.
This also works if you go to Options & Career, Player ID, and edit your Clan tag. Just type in .fortune, hit enter, and you'll still get the achievement.