I am running Win 7 on a test machine and I can see network targets through Win7 backup. First time using it however I simply clicked start, typed in backup, clicked, Back up my computer.
Next I clicked configure backup (since I had never done it), once that opened up I saw my C drive and my DVD Rom. There is a button int he lower right hand side just below the list box with HDD/Dvd-rom that says "Save on a network". Click that button, click browse and find your DNS-321. If you can not find it through that list try typing the address manually like, \\IP_ADdress_321\Volume_1