After I joined the Partner Program, the option to “Make this story eligible to earn money through the Medium Partner Program” appeared near the top of my posts. The format has changed a little over time, but currently that option looks like this:
If you click on it after you’ve written a story, the following question will appear:
“Would you like to make this story eligible to earn money?”
You will see two radio buttons beneath that question, so all you need to do is click the one that says “yes.” :)
That said, it is my understanding that Medium has not yet been able to establish payment systems with users whose accounts are based in certain countries. In those cases, unfortunately, even though Medium will take the subscription fee and unlock all stories for that community member, Medium will not be able (supposedly) to pay that community member for their own stories. (You can read the informative post that Sarah Nderi wrote about being barred from receiving payment here.) If you think that your location might be a factor in why you aren’t able to receive payments yet, perhaps you can write to a Medium support person just to be sure.
I hope that, whatever the issue is, it can be resolved and you can start earning money for your hard work soon!