Undefined symbols x86_64 error XCode

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Hi,

I've been trying to compile a hand detection example I found and I have been getting the following error:

Undefined symbols for architecture x86_64:
   "_cvCvtColor referenced from:
       _main in main.o

The weird thing is, when I comment out the cvCvtColor() member, the code successfully compiles! I'm using XCode 4.6 on Mac OSX10.8 (Mountain Lion)

Any insight is really appreciated!