Before you'll be able to export & publish to a Medium site, you'll need to follow a few simple steps:

Step #1: Click on Connections

Step #2: Click on the Medium icon

Step #3: You'll be redirected to Medium, where you'll be asked to authorize your Wordable connection. Simply click on Okay and you're all set! Once you've done that, you'll be able to see your Medium connection listed under the Export Connections section.

Here's a quick video on how this process should work: