Spent a few hours working through this tonight and kept getting the error "The folder you entered does not appear to be valid" after repeating the username and password twice. I have WebDAV enabled via HTTPS so was using https://xxx.xxx.xxx.xxx:5006/ internally. I tried modifying the registry mentioned in this post http://is.gd/ki9tMz but it didn't make any difference.
Finally I tried mapping the drive using the network name [duh] and it worked first time. Just ensure your user has privileges to access WebDAV under Control Panel - Users - username - Application Privileges as shown below.
DS209 DSM 3.2-1944