$this->ftp->mkdir()

[eluser]tomdelonge[/eluser]
Codeigniter User Guide - FTP Class

Perhaps there's something wrong with server permissions.

for the mkdir() function, it says:

Lets you create a directory on your server. Supply the path ending in the folder name you wish to create, with a trailing slash. Permissions can be set by passed an octal value in the second parameter (if you are running PHP 5).

I can't think of anything else. Good luck.