First of all i like to mention that i don't have used xFire before but i finally got it working.
GBStudios is right, you need to register your game / project at xFire before it's recognised by xFire.
I've copied the sample plugin from the Unity page and build a small C++ plugin for Unity. Actually it's just a wrapper dll for those functions the xFire SDK offers to you. I've packed the C++ project, a sample scene and a C# interface script into a zip and put it on a free file hoster. (btw. can you upload files in a better way to include them here in a post?)
I used the UTF8 version of xFire to set the string data. I'm not sure if the ASCII or WideStr functions will work but the UTF8 works fine, even with special characters.
You can trick xFire by replacing a known game with your Unity-build. I used the minesweeper for testing ;)
Before you can use the SDK, your game has to be detected by Xfire. For more information on how to add support for your game in Xfire, please send a brief email describing your project & and your position with the team to: