As a matter of fact, setting the license key on the Xceed.Ftp.Licenser.LicenseKey should normally unlock the Xceed.FileSystem too. You should look for a line like this one:
Xceed.<b>Ftp</b>.Licenser.LicenseKey = "SAMPLE-APPLICATION-KEY";
a) Make sure this line is called at the very beginning of your application, before you access any FileSystem objects.
b) If you are working with a trial package, make sure the license key that appears in the Xceed Component Licenser isn't expired.
c) If you have a registered "FTN" license key, replace "SAMPLE-APPLICATION-KEY" with your registered key.