Since you're working with a NAS, FileZilla server is not an option. The NAS is self contained with it's own OS and no means to install apps (unless developed specifically for that device). Once FTP is enabled, an FTP client is not required. You should be able to navigate to the NAS by entering the FTP address in a folder window while in Windows Explorer.
I looked at the WD Book Live user manual and the device seems to only have an "FTP Enable" checkbox, but I did not see any settings for FTP (unless the manual was just written too vague). The question here is, does the WD Book Live have an internal FTP Server? If so, there should be options to add user accounts, set permissions R/RW, and identify directories for sharing.
Are there more FTP settings than the manual illustrates?