I believe there has to be an action taken from the Facebook side before it'll appear in this list. That's how it works for Linkedin and even ContentAI -- without the first lead, the list will remain empty. It's a bit counter-intuitive because you can't setup your flows until after a lead should have gone through it -- but that's where we are.
We're experiencing the same issue this morning- we were able to get a few lead gen forms in the drop down originally, but we made some changes and now it won't populate. We filled out the form hoping it would populate a lead but it didn't. Did you get help for this?
After a lead ad form is built in Facebook, you should see it as an option in Marketo (assuming the integration between the two systems is working). I've experienced this before and had to wait several hours (and submit the form with some test leads) before the Facebook form appeared as an option in Marketo. Another thing to note is that many people are experience Facebook to Marketo integration issues as noted here: Facebook Lead Ads not capturing leads
I have an open ticket with Marketo now as our Facebook integration has been down for three weeks. That could be another part if the issue!