I see the issue, and not as intuitive to fix as one would think.
Under Item Detail, select the Visualization Tab and Not the Data Tab.
Then, ignore the "Fields" button, doesn't seem to do anything but let you change the alias for them. However, select the Popup config button, Select Fields list, then "Select fields" and make sure all the fields you want to show have a check next to them, then select Save in upper left.
Now when you go back into the Data Tab, you will see the fields that you turned on in the Popup config of the Visualization Tab.
R_