Load up your game first. Then click Cheats>Cheat List. You should see 8 buttons along the bottom: Code, Cheat, Gameshark, Codebreaker, Remove, Remove All, Enable/Disable, and the Ok button. Six of those buttons should be clickable. The ones you will probably want to use will be Gameshark and Codebreaker.
The codes that can be entered into under Gameshark are codes using this format: XXXXXXXX YYYYYYYY
The ones for Codebreaker are: XXXXXXXX YYYY
Note that the codes that can be entered can be something else other than Gameshark or Codebreaker. Long as they use of those two formats, they can be entered. Just choose the format that matches and viola.