These face recognition algorithms are available through the FaceRecognizer class in OpenCV's contrib module. Due to dynamic linking, it is possible that your program is linked to the contrib module but it is not actually loaded at runtime (if it was deemed as not required). So it is recommended to call the cv::initModule_contrib() function before trying to access the FaceRecognizer algorithms. This function is only available from OpenCV v2.4.1, so it also ensures that the face recognition algorithms are at least available to you at compile time: