[MikeDubreuil]

I think you should simply explain the situation. Explain the registration system allowed you to register and they you did so because you knew your team wanted to go.
Then FIRST told you that the registration system made an error and you should not have been allowed to register. Make sure you explain the reasoning behind rookies not being allowed to register so they are not discouraged.
The reasoning behind rookies not being allowed to register for nationals is purely based on a capacity problem. The rule is simply to give teams who have been in the program for many years a better chance at being able to attend nationals.