Did you already follow these steps from the README?
Once this is done, you just need to add this to your project.xml
<haxelib name="extension-googleplaygames" />
<setenv name="GOOGLE_PLAY_GAMES_ID" value="32180581421" /> <!-- REPLACE THIS WITH YOUR GOOGLE PLAY GAMES ID! -->
Also, you may need to set android sdk version to 23 or higher (as some versions of google play services requires this:
<android target-sdk-version="23" if="android" />