Just found a level 5 keypad in the first room Jensen wakes up in Singapore. It is the keypad to the small room in the northwest of the room he wakes up in (Omega Ranch). First walk up to the keypad and save the game. Then hack it with or without nuke virus to the red server thing. Immediately after it shows your rewards DO NOT PRESS X OR EXIT THE SCREEN, instead press start and load your last save. Repeat this process of hacking and loading last save and the rewards of the nuke virus will stack until you finally press x to reap your rewards.