The Wii's choice browser; Opera; is rather... poor to say the least. If Opera is still supported (hell if I know, I've never used it lol, doubtful I'd ever want to), the Wii isn't going to get new updates any time soon. Sorry to say, but there's nothing you can do to get Pandora working on your Wii through the Internet Channel.
Nothing official, anyway. Maybe some diehard Pandora fans made some unofficial patches you could find or somethin'... Doubtful though.