To add a new table to your interface, click the "Add" button found in the tables section of the configuration page.
If the table you want to add to your interface has never been used in an interface before, it will not show up in the list of tables when clicking the "Add" button. In that case, you'll need to do the following:
Click the "Add missing tables" button towards the top of the table picker.
Select the table(s) you'd like to add.
Click "Add selected". This will add linked record fields to your selected table(s) pointing to the miniExtensions table in your base and add the table(s) to the interface.
Optional, but recommended: Hide the linked record field pointing to the miniExtensions table. You won't need to do anything with this field, so it's best to keep it out of sight to prevent any accidental changes. Changing or removing this field while still using this table in an interface would break that interface.