I understand the issue you're having and will try my best to assist you in this matter.
Since you have already installed the WPForms Multilingual plugin on your site, you should see the WPForm post type in the translation management. Since it is not showing, maybe the Form was created prior to installing the WPForms Multilingual plugin.
Please open the Form and make any small changes to the form's contents. You can either slightly change the field name or make any small adjustments and save the changes.
After saving the changes, please go to "WPML > Translation Management" and hopefully, you'll see the WPForms post type in the list.
Please let me know if that helps or if you need further assistance. I will be happy to help.
