To add a schema, you must create a new data source and associate it with that schema. On the Data tab, click the "Add Data Source" button to add a new data source. As part of this flow, you will see a 'plus' sign next to the schemas dropdown, which you can click to enter a schema name. This data source will automatically be assigned as the default data source for your new schema, and you will be prompted to sync tables on connection to the data source.