I have attached a link to the manual. On my xfi titanium Fatal1ty card it is the blue socket. I assume it is the same on yours. It is next to the green output socket on the end.http://support.creative.com/manuals/
If you your mic isnt working and its in the correct socket, open the software for the card and double check that the blue port is set up for mic and not Line-in.
The easiest way to set this up is through the Creative Consol Launcher application or the Alchemy application. If you do not have that software on your PC, it can be found on Creative's web site.