In order to open the setup menu click on the Sites tab from the top-side menu and choose the Site Manager option.
Extra attention should be paid to the port and connection protocol combination. Those can be set in the Port field and the Connection drop-down menu.
The correct ones are:
21 port for FTP:
For setting up the starting folder of the FTP session when you use username, click on Advanced menu which is located in Site Manager window and then select Directory/Folder option from the left-side list.
In the Remote Start Folder input public_html if you want the FTP client to start from the main website's folder:
You can check the FTP Client Error List here.