plz make sure that lib and jars are all in your project:
~/repos/sdkbox/sdkbox-sample-admob/cpp/proj.android [master] $ ls jni/
Android.mk Application.mk hellocpp pluginadmob pluginsdkboxads sdkbox
~/repos/sdkbox/sdkbox-sample-admob/cpp/proj.android [master] $ ls libs/
PluginAdMob.jar admob-chartboost-adapter.jar android-support-v4.jar gps
adcolony.jar android-async-http-1.4.8.jar chartboost.jar sdkbox.jar