i think i found it..is it in community mode?
No, it's not possible in Community Mode, in fact it's not possible in any mode to be able to play a tournament with some mates and choose to play as your mates opposition CPU team. Absolutely ridiculous that the developers have removed this option on the current gen versions of PES (it's always been available on the PS2 versions). It's ruined the game for my mates and me. I've ranted on about this a lot!
http://www.pesgaming.com/showthread.php?t=71134
See also posts 24, 25 and 26 in the comments on Dan's Interfering With Play column on today's home page.
They really should bring this option back. Half of my mates have defected to FIFA exactly because of this problem.
EDIT: If you have the PC version you are in luck. There's a patch, called the Kitserver patch that fixes this problem, simply install it and click "Disable Controller Check", and simples, it's fixed. If you have the PS3 or Xbox 360 version then you are stuck with this problem, like me. This is a link to the Kitserver patch (from December, so it's probably updated somewhere else in the editing section!)
http://www.pesgaming.com/showthread.php?t=73671&highlight=kitserver+controller+check