Go to Superset and choose Data -> Datasets and click on the + Dataset button in the top right corner.
Fill in the Add dataset modal as follows:
Database: Choose the connection that you created above
Schema: Select druid
Table: Choose the table that you want to query from the dropdown menu. This dropdown menu should show the same datasets that are displayed in your RIll workspace. Each dataset represents a table.
Now that you've created a dataset, you can click on it under Datasets and edit it (under Actions, over on the right) to configure column properties such as whether the column is a metric or dimension or whether it is your time column. See Superset documentation on Creating Your First Dashboard for more details.