The ability to send a mugger is unlocked when you reach level 50. If you are going for the achievement, obviously you don't need to be at that level, however if you are boosting it, you must know someone who is at that level. Alternatively, find a session with someone over level 50. Either ask your boosting partner or irritate the other players in the session, and hopefully they will send someone after you.
Despite its rarity, it is actually quite a simple achievement to unlock. Once someone has dispatched a mugger after you, you must get up off the ground and kill the mugger. Obviously a firearm is the easiest. Once he is dead, it is the simple case of walking up to the dead body and reclaiming your money.
It should be noted that you need to have money in your possession - essentially, you cannot have all of your money in your bank account, as the mugger will have nothing to steal, and thus you cannot reclaim your money.
I have included a video guide which explains how to unlock the achievement as well. It was unlocked on the Xbox 360 version of the game, but the process is the same.