The thing is Wattpad hasn't made a widget that lets you post a cast list from the mobile app. So you have to do it on the full site.
Use safari or whatever, to go to the actual website, click on view full app and then go to create.
Edit your story and click on the three dots at the top right hand corner of the page.
Then choose your cast.
Make sure you publish the changes before exiting.
And you should be able to see your cast list on the mobile app.
I hope I was helpful.
—Cookie. :)