So you installed Civil 3D as a networked version, not a standalone version, correct?
And you are putting in the license server's IP address, not your local machines, correct?
Sorry for all the questions, but I always create deployment images and do network installs via vbs, so running through the standard Install Products procedure isn't fresh in my mind pertaining to prompts and request for license servers.
However, if I remember correctly, all you should need is the IP address to the server the FLEXLM and license are installed on. It wouldn't hurt to try the actual name of the server too just for giggles.
Assuming you aren't the one that installed the FLEXLM, maybe you should call that person for support. They know their system and will be able to solve this issue quickly.