Just take your passport, I think you have to go upstairs still. Tell the lady on the desk that you have a vergi number but no card and they will give you another card. In Turkish "Benim vergi numerasi var ama kart yok. Bana yeni kart lazim lutfen". My Turkish isn't brill but that got my friend his card when I went with him.
I would also take a photocopy of your passport in case she says you don't have a number, she will then give you one. No problem and no charge for this.
The tax office is on a little road between the Migros road to Calis and the New Kordon harbour road. It is sign posted as the "Maliye"