If its a Cpanel account and anonymous FTP is enabled, You'll see a linkin the FTP Manager area of the control panel, if its there, you'll be able to
setup the welcome message, allow uploads etc.
Have you tried logging in as anonymous@yourdomain.extension?
Replace yourdomain.extension with your real domain and extension.
If you click on Add/Manage FTP accounts you will see a few
already made for the main account by the system (not deleteable)
they usually are ftp.yourdomain.extension and anonymous@yourdomain.ext and
possibly some log directories where you can ftp to download your access
logs for your website.
Also, if your account has the capability to add other FTP accounts, remember
that only the main account can use the cpanel username and password for
ftp.yourdomain.extension, anyone else has to use their full email address
as their username and their assigned password.
I believe that the reason this error occurs in a Cpanel system that has
anonymous access enabled is that a user named anonymous already exists
Anyway, I hope this helps you out.
JB