I know I'm breaking the rules of this thread talking about stuff like this instead of flight and tracking information, but hopefully ya'll will forgive me this one transgression.
If you want to migrate a user from your old machine to your new machine and you have already created that user on the new machine you can do it this way:
Create another user on the new machine (System Preferences->Accounts)
Make sure the new user is an Admin on the machine
Log into the new user account on the new machine
Delete the user name you originally created on the new machine
Run Migration Assistant and migrate over the user
It actually takes a lot less time to perform these steps than it does to read about them so if anyone finds themselves in this position it's a relatively quick and easy thing to work around.