There is no longer a separate approval process for Marketplace native ads.
To be approved for MoPub Marketplace access for any format, including native ads, please log into the MoPub UI, navigate to the Marketplace tab and enter your company and payment information in the MoPub UI.
Next, you'll want to email the MoPub Policy Team, who handles Marketplace approvals, with up to date payment information and the corresponding Android package name/iTunes App Store URL/Website URL linked to your MoPub App in the MoPub UI. For faster approval, please include screenshots of the app in your Admin page/your website's admin page to show that you are the owner of the account in your email to policy.
The Policy Team can be reached at email@example.com.