I don't know anything about setting up a private server, or which files you need to modify. However, I can tell you where to find the folder on your Mac which corresponds to the C: drive on Windows. From CrossOver's Configure menu, select Manage Bottles. In the Manage Bottles window, select the bottle you're interested in. Select the Advanced tab. Click the Open C: Drive in Finder button.
Another way to find the same folder is to navigate in the Finder to:
(your home folder)/Library/Application Support/CrossOver Games/Bottles/(bottle name)/drive_c