From my experience there's no way to specifically advertise on an app, when I see my facebook ads stats on prosper every once in a while there will be a link from apps.facebook.com so it seems they just dish them out randomly...
There are actual networks (like social media) that you can use to advertise on some apps, but the apps have to be participating in that program. Otherwise you'll have to contact the apps owner.
Probably if you did a little fucking searching, like gee - I don't know - actually go to the fucking Mob Wars page on Facebook:
Mob Wars | Facebook
And I don't know, fucking looked under the 'About Mob Wars' header, you'd see a link to e-mail them. Just a fucking thought.
I found that literally in under 30 fucking seconds thanks to Google.