I had to set up Facebook Business and Audience Network.
Then followed all the instructions and the tutorial on the Gdevelop Wiki.
http://wiki.compilgames.net/doku.php/gd ... s/monetize
The bit that I found tricky was setting up test ads.
I had to find a number on my mobile phone: An AAID I think it was, and enter that on the Facebook developers setting page.
And that allowed the Facebook ads to be tested on my phone.
That whole process took me a whole day, but I'm so glad it finally worked.
Let me know if you need help with any of the steps.