After installation, the check box column needs to be added to the message list. To do so, click the column picker button on the far right of the label bar to see a list of the available columns. Alternatively, you can right-click on any column label to display the list. From the list select "Selected" to add the check box column.
You can toggle the selection status of a row by clicking/tapping the circle. By default, the row height is increased to reduce potential tapping errors. This can be turned off in the extension's settings.