I have searched but can't find the answer...probably because it is so basic

I have set up transmission on my computer running ubuntu. I have a NAS that I use (ReadyNas Ultra 2). I can move files to NAS fine.
I want Tranmission to download files directly to the NAS but I can't browse anywhere except local to computer when I try to do this from Transmission menu. Am I missing something really basic?
I have installed Tranmssion on the NAS too and that works OK at first, but then stops the downloads after a while as the latest version on NAS has a memory leak, so can;t use that either.
Thanks