Are you sure you are using the user/password for the user you setup and NOT the username/password you use to access the NAS via the web UI?
Did you also allow that user the necessary shares?
Did you setup Samba/CFS and/or FTP for that user?
Above all did you save your settings at each stage?
Sorry if this is old hat but those are the most common things that stop people logging in to a user A/C.