If your router lets you configure ftp but you can't access it, could be a firewall issue. Could also be a port forwarding issue. No offense intended, but a little more info wouldn't hurt. Like model and firmware. Have you setup ftp before without a problem? Just trying to help.
Yes, more info would be helpful. FTP can cause problems with NAT common devices. Are you trying to run an FTP server or are you just trying to download files from some random FTP site? Are you using PASV connections? Skimming some Buffalo router manuals, it looks like they have a built-in FTP server on linkstation/airstation devices.
It's not quite clear what you're after. Few examples of questions which you do not provide any information: - By "FTP", do you mean client or server? - If server, is it located on LAN or by router (with USB hard disk)? - Does your WZR-300HP have DD-WRT or Buffalo firmware on it?
I'm still unclear as to the context of the failure. How about a screen shot? At least that would be a start.