subdomains

lol - you have Cpanel right?
I have the same thing - just add the subdomain - then go to ftp accounts and make a new account - user, pass blah blah -
you should have something like that in your "add new ftp account" section -

Directions:
If you enter a single / in the directory box, the new FTP user will have access to the entire public_html directory and all directories under it. If you just want the new user to be able to access a sub-domain, just set the directory box to the same value as the sub-domain's root name, which is listed in parenthesis on the sub-domain section of this control panel.

just create a subdirectory that the new ftp account will be able to access - the same name as your subdomain maybe....

[edit]= or just do what zyc said - call your webhost - they should be able to tell you all the details

i have cpanel

u create a folder off ur site so it will be www.domain.com/foldername

and then u creat an ftp account with a user name that is exactly the same as the name of your folder