Ok, update. I've gotten the extension working now. What was missing was the fact I didn't fill in 'Mediation settings' on HeyZap.
Now that's all good. I'm getting a few more issues for showing ads (specifically reward ads - ones that you can't skip).
I'm currently using Chartboost, AdMob, and Ad Colony to show my video/interstitial ads.
Ad mob - works well, but unfortunately does not support video reward ads (unless those new universal ads are the ones I'm after, but again, dont believe it's supported at this time), so that's why I need to rely on either Chartboost or Ad Colony.
Chartboost - extension is currently out of date, and I don't believe Rob can figure out how to update it :S. Another problem is that chartboost's extension causes compile errors, but that could be to do with the issue Rob has been having.
Ad Colony - another one which seems to be out of date, and which I cannot use video ads for. The extension (which is active) doesn't cause any problems, however it's saying on the Debug that:
No SDK Detected
AdColony - ??
So at the moment, I'm unable to use video reward ads unless I sign up for another Ad group. If that's the case, any suggestions?