the basic minimum dress code for women is a skirt and blouse (basic business attire) which the costume bassically has. if you have any way of dressing it up a little, i'd recommend it but it works more or less as is.
if you don't really want to go, tell your friends. if they're really your friends, they'll understand. there are always other things going on during the con so even if you don't come to the ball, there are plenty of other things you can do while they're at the ball. there are the anime rooms, karaoke, stage zero, panels, etc. and, if all else fails, just remember that you're at a party with 15,000 of your closest friends. everyone at the convention has at least one thing in common - they all like anime. i guarantee you can find someone to chat with and who knows, maybe you'll make a new friend.