XCode and C++ linking

Hello,
first of all, if there is a better place to ask developer-questions concerning xcode and c++ please tell me 😉

I am currently working on a c++ project and I am using XCode 2.3.
The problem is the following:

I have a folder for my project files and another folder where other c++ files are stored (a whole big package). Within my files, I am including files from the project-external folder. The include itself is working, because it recognizes the classes and their methods, etc.

Nevertheless, there seems to be a linking problem (ZeroLink is turned off):
Linking (1 error)
Undefined symbols:
Tribots::ConfigReader::append fromfile(char const*, bool)
etc. etc. etc.

I tried to explicitly include the external files in the "Linking Binary with Library" section, but afterwards I had the problem that the used paths were related to the external folder and not my project folder.

How do I manage to get XCode link all needed files (the included and used ones) right? It does not even seem to compile the included .cpp files ;(

By the way: I tried to compile the whole thing using a makefile and it worked, so the error has to be somewhere within my XCode configuration..

Thanks a lot for your time and help,

Greetings,
Tim