Extending the Meta Data for your Pages
First, in order to add your own fields to the metadata base fields provided by SCORE, you should create your own data template (something like Meta Data Extensions), and add the new template into your page types for the tenant site.
Next, make sure to use a field section named "Meta Data"
The field editor button on the Page tab does two things - first, it is only enabled when the current page inherits from the SCORE base template called Page Meta Data. Second, the field editor finds and shows all fields in the popup dialog that are within the field section "Meta Data" - so if you add your own fields there, the dialog should include them.
Next, you will need to create a rendering to render the new fields you've added to the META Data. You can use the SCORE rendering as a reference, but you'll need to create your own.
Once you've done that, you're set - the final step is to add your new rendering to the layout document (or however you plan to add it to the page, do that).