OK, if you want to use it like a regular drive letter (which is really a microsoft thing) you need a couple more programs.
NetView 2.90 -
http://www.tucows.com/preview/360750 (use Download Now button)
and
FTPDrive 3.5 -
http://www.killprog.com/fdrve.html (download link is at the top of that help page)
To make NetView work better, give your router a 'Host Name' if you didn't already, on the Setup->Basic Setup tab.
Save and Apply.
Install and start NetView.
Right-click its icon in the tray and choose Open.
If the router is not listed, right click at the bottom of the list and choose Add host.
Put the IP and hostname in...
Leave everything checked that is already and add a check next to FTP Active.
On the Advanced settings tab, enter a username and password as you entered in the router on the Services->NAS tab. Check the Open path select
ftp://%HOSTNAME% and add a / on the end (so it says
ftp://%HOSTNAME%/ ).
Click OK.
Use File->Save hostlist. Click the minimize icon in the upper right corner (it minimizes to the tray rather than the taskbar).
Install and start FTPDrive.
Right-click its icon in the tray and choose FTP Drive settings.
About the only thing you might need to change is the drive letter. The default is Z, if Z is unused.
Now you should be able to open Explorer or any program that needs a drive letter (Explorer does not *require* that, by the way... it can FTP to the drive without using NetView and FTPDrive) and access the drive by letter instead of FTP only.