You must edit a game file to activate this cheat.

First create a backup of the file. Edit the "metal.ini" file which can be found
in the "system" folder inside the game folder. Find the profiles and look for the
line that says "Cash=" followed by numerous characters. Change those characters 
to the following and save the file to get the desired amount of money: 

$1 Million   : 5qqgFPfZOrOA6oD9BqPcHf2)+YA 
$15 million  : Tfp58kEZ8xy8GrnbJ4kSpBvtaq8 
$999,999,999 : 9E!4q7fYKoZWNUxfup7GNuQSsT+