Adding a header file to Xcode

[n00blar]

First off, the following question was posted already, about a few months ago in these forums; however, I seem to have a slightly different error message, and I can't find the answer for this issue in the place where I got this header file.

When building the project, I get the following error message:

Undefined symbols for architecture x86_64:
"_GetString", referenced from:
_main in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I installed these files using the following commands:

gcc -c -ggdb -std=c99 cs50.c -o cs50.o
ar rcs libcs50.a cs50.o
rm -f cs50.o
chmod 0644 cs50.h libcs50.a
sudo mv cs50.h /usr/include
sudo cp libcs50.a /usr/lib


My version of Xcode is: Version 4.5.2 (4G2008a) and I'm running OS X 10.7.5.

I have no clues as to how to get this to work. Thanks for any suggestions.