keep playing
you have to have gold in all the cups to unlock it :)
you have to do time trial races to unlock characters/vehicles
to unlock dry bowser, you need to get a one star rank or better in all 150cc Wii cups (all the cups on the top)
to unlock everyone you have to play grand prix all classes and to unlock the rest you must do time trials it takes a while but its fun
all you have to do is get a gold medal in all the other races of all the other cups, then you should unlock mirror.
i think you must beat 50cc with all gold if you do this you will also get King Boo
get 1st place in all the other races and i would recommend Yoshi to race with (just to unlock stuff though)
you have to complete certain things to unlock stuff. this website shows you how to unlock everything, including karts. http://www.videogamesblogger.com/2008/04/18/how-to-unlock-all-mario-kart-wii-secret-characters-bikes-karts-and-more-guide.htm
To get mirror cups in Mariokart Wii you willk have to complete all of the cups on all engune sizes
There is only 1 to unlock and that is Toadette the pink toad. To unlock her you have to at least do all 32 courses in time trails (but you dont have to win them)
to get mii outfit 2 you have to unlock all expert ghots on the time trials
Play Time Trials on all 32 different courses or win 1000 WFC (Wi-Fi Connection)matches.