Col. Turner,
For now, you don't. The registration system is in the process of being updated but there have been technical difficulties. Just keep track of the game details. The update will eventually be accomplished and we will all be able to register our Vicksburg games after the fact. (I'm engaged in a Vicksburg battle right now myself and I'm in the same situation you're in.)
Your humble servant,
Gen 'Dee Dubya' Mallory
David W. Mallory
ACW - General, 3/2/I/AotM (Club President & Cabinet Member)
CCC - Ensign, Georgia Volunteers, Southern Regional Department, Colonial American Army