I don't have expertise with the NDK, but as far as integrating the SDK with Eclipse and getting started, I could walk you through the process.
If you have downloaded Google Play services, you will find the framework split into smaller .aar chunks, each corresponding to a separate SDK from Google (Maps, Locations, etc.). Grab the Ads portion, and use Gradle to integrate it into your project in Eclipse.
Using Gradle will significantly simplify the process so that you don't have to modify the .aar files manually. I have found the following links to be helpful in getting started:
While this process requires installing extra plugins into your existing IDE environment, it is a more bullet-proof way to get the aar integrated. Should there be any error in the process, you will be notified by Gradle.
If you do decide to upgrade to Android Studio, simply input
compile 'com.google.firebase:firebase-ads:9.4.0' in Gradle to download and integrate the SDK.
To load and display interstitial in your C++ project, check out
this tutorial.
Let us know if you have any questions,