The ad's will not start showing up immediately, give time for testing. Another important thing is that the ad's are shown according to availability in mopub; if the problem is your fillrate, there's not much that can be done, besides adding more networks and increase the number of requests with more downloads.
Mopub is a bad tool, but unfortunately we have to use it because it is the only option at the moment.
I believe Ludei and Mopub have some sort of business, because otherwise I think there would be support for some networks, directly.