Here's a handy little guide to hacking in BioShock.
Hacking
With all the machinery and brilliant minds in Rapture, it's not suprising that someone eventually figured out how to reprogram the machines with Hacking.
By successfully redirecting the circuit flow, you can change the current state of the machine. The ability to Hack is present on almost all the machines found in Rapture. Hacking can open locked safes, and can even be used to reprogram the security system to view you as a friend, and your enemies as the intruders to be attacked. If a machine can be Hacked, a prompt will appear when you are next to the machine. Pressing V (or X on the Xbox 360) will bring you to the Hacking menu that will allow you to evaluate the difficulty of the Hack as well as whether you want to Buyout or use a Autohack Tool to complete the hack.
How to Hack?
During the Hack, the object is to redirect the circuit flow by uncovering tiles and swapping the pipes you need to move the circuit flow to the exit tile on the board.
Watch out for hazard pieces that can increase the speed of the circuit fluid, cause an electrical overload, or even sound a security alarm. The benefit of succeeding is that you gain the benefits for free, but the risk of physical damage is high.
If you are finding Hacking is getting very difficult, it is time to spend your ADAM to upgrade your Engineering track with more slots or to purchase more engineering tonics.
Hacking Buyout
Hacking can also be achieved by paying a premium price. You can spend your money to override the machine. The more difficult the hack the more money it costs. Splicing Engineering Gene Tonics can reduce the buyout costs.
Autohack Tool
Finally, if you are lucky, you can find Autohack Tool that immediately hacks the machine with no issues. When at the hacking evaluation screen click on the Autohack button to immediately succeed at the hack with no risk. Autohack Tools are used in the process.