You can select a folder by using the radio buttons along the left column. Once you have done this, select the action you wish to perform on the folder via the select box to the left.
You may be prompted for further options. Be careful with the delete option, as you can lose all the messages in a folder if you delete it by mistake!
This option will import mbox format or .eml files into the folder of your choice. Choose one folder to import the messages into and then select this option. You will then be taken to a page that will prompt for the mbox or .eml file to import.