When you FTP into a ProWeb account, you are "jailed" into the directory specified in the configuration for that FTP account. For example, if your username is "foo", then you have a default FTP account with username "foo" that is jailed into /home/foo directory.
Jails for FTP accounts are a good thing for lots of reasons. The problem in this case, however, is that the web logs reside outside your /home/foo directory. In fact, they are in /var/log/httpd/users/foo.
So the trick you need to access your logs with FTP is to create an FTP account on the members page, call the account username "logs", with the jail directory set to /var/log/httpd/users/foo.