i had this exact thing happen to me. i have the gamecube version so i dont know if this is different for the ps2 or not.

start by setting up a game, picking teams and whatnot. After the cointoss pause the game. The screen kinda gets split in 2. on the right bottom beneath the quarter, down, etc... it says team control. that should be on user not CPU, next to it should be a symbol with the button to use.

that worked for me, but again i dunno if ps2 is different