Build Failed sdk.dir is missing in ofAndroidLib

Hello
I followed the instructions in http://www.multigesture.net/articles/how-to-setup-openframeworks-for-android-on-windows/

when I run Android Install on example projects (android polygon), I receive the following error:

**BUILD FAILED
D:\Android\android-sdk\tools\ant\build.xml:593: The following error occurred while executing this line:
D:\OF_dev\addons\ofxAndroid\ofAndroidLib\build.xml:46: sdk.dir is missing. Make sure to generate local.properties using ‘android update project’ or to inject it through an env var

Total time: 1 second
make: *** [AndroidInstall] Error 1**

How can I solve this ?