make a search on the forum and have a look at the Knowledge Base
http://www.ioftpd.com/kb/view.php?kbid=98
http://www.ioftpd.com/kb/view.php?kbid=113
and try to follow this instructions:
http://www.ioftpd.com/kb/view.php?kbid=77
after all .. ask an other time specifing exactly what is your configuration!