Hi. I followed these steps and works fine.

1 Import the face-detection sample and OPCV Library

2 Remove the wrong link and add a correct link

3 Put the full path of OpenCv.mk (in android.mk)

in my case: include E:\OPEN_CV\OpenCV-2.4.11-android-sdk\sdk\native\jni\OpenCV.mk

4 Configure environment for NDK

5 Configure Build

PS.: You need a NDK. You can download here

That's it.