Select the column next to where you would like to add a column
Right-click somewhere in the column and select either "Insert column right" or "Insert column left"
Congratulations, you've created a new column! You can now add data to it.
Once you've added data to your new column, make sure to add your new column to one of the boxes in the Select columns to visualise panel. The new data won't actually appear in the visualization until you do!