To be able to see a Submitted Forms tab you must be in User Group with the correct User Permissions. 

All Submitted Forms will be in the default All Forms tab. If you create a custom tab and cannot see a form in that tab it could be that you don't have the user permissions to see that particular form or that the form template is not selected to be available in the tab you have created. 

To add a custom tab > Web Portal > Forms > Submitted Forms > + New/ Edit > Add in the Title > Select the Form/s you would like to show in that tab > Select Groups you want to be able to view the custom tab > Add in any columns you would like to have > connect the form object name to the column in which you would like that detail to appear > Save.