Generally Root logins are fully denied via FTP unless you force them to work. FTP authentication and transfers are done in plaintext. There's no encryption of your root password as it's transmitted at all. This is why all ftp daemons deny root login by default.
You *can* enable on some FTPds, but it's generally a really really really bad idea. If you truly need to access files as root on your server, you should look into an ftp client that supports SCP/SFTP (essentially FTP tunneled through SSH). They'll let you do what you want, without the security issues involved.
I'll second that - enabling root logins on any protocol is a bad idea, on one that uses plaintext authentication it's just plain madness.
We're going to need more information to debug your ProFTPd problems - what error message are you getting out of the FTP server? 530 Login incorrect?
You can probably glean a bit more information from looking at proftpd's log files - on Ubuntu these are normally found in /var/log/proftpd - if you paste the last few lines of your log file from when you're trying to login we might be able to help further.
Agreed, we'll need to know what's in the logs when you try to connect. Now what *should* be happening here, is that when you add a user via webmin, the user is then added to /etc/passwd and /etc/shadow. Now ProFTPD should look at that file for authentication.
From the commandline you can do 'grep <username /etc/passwd'. Verify that the home directory mentioned in the password file actually exists as well. If you don't find it in the password file, well then that's your problem.
However keep in mind, this is just basic speculation and without knowing what's in the logs, it's simply that, speculation.