Right now, the Zapier WordPress app doesn't support custom fields within posts, however, you should be able to get them to show up by using the Wordpress plugin, here. After installing the plugin, try creating a new post with custom fields and they should come across for you. Thanks to our awesome user Dave for this tip!
Note: This is a potential workaround suggested by a user of ours and therefore, we cannot help with troubleshooting in regards to the plugin. If you installed the plugin after creating the Zap, try creating a brand new Zap if you're having trouble getting them to appear.