NOTE: Some UI elements in the Dataset Management Experience have been updated since 12/7
In order to add a new column to an existing dataset, you will first need to create a revision of the dataset by clicking on the Edit button using the action bar from either the Primer Page or the Data Table:
Select Review Data to open the data preview and configuration modal:
You can then add new columns to your dataset by clicking on the "Add Column" button on the left-hand side of the data table preview. In this modal, you can choose the Display Name, Field Name, Description, Source Column, and Data Type of the new column.
NOTE: Valid field name characters are all lower case, alphanumeric and underscore. It must start with a letter or underscore and cannot contain special characters or spaces.
Click the Add Column button in the lower-right corner of the 'Add column' section, then navigate back to the data preview by selecting Done at the bottom right.