In Mainmenu go to ''Extras'' than ''Codes'' and you can type in the passwords. If correct, it will confirm at the bottom of the screen.

Password - Effect

GM2OPEN8 - All Gamemodes
A11T3AM5 - All Teams
MAXSKILL - Max. Skills
Z26BEXW8 - Micro Soccer
E06J3CT5 - New Object in Inventory
SPEEDY01 - Unlimited Turbo
5Y104D9A - Unlock Streetballerteam
WIMP_A.1 - Weak Opponents