You’ll need to create the mount point on your Pi first (usually these are a directory inside the /mnt directory.
Once you’ve done both of those you can run sudo mount -a to mount it and then point Transmission to use the directory you just created as the mount point. Anything written to that directory will be written to the NAS.
ehm, dude, you are asking about NFS, but you’re using CIFS (aka SMB, aka SAMBA). This are two completely different network protocols. CIFS is much more easy to use for beginners and much more easy to maintain. And it’s allways implemented into almost all NAS devices, as well as in every Windoze since Win 3.11 NET. NFS has one major advantage: it is integrated to the folder tree of your mini linux box “as if” it would be a regular hard disk or SD card. Meaning: you can see how much spoace is left on your NAS disk if you take a look into system satus screen of your OSMC box. But on the other hand, it is a pain in the ass to manage if you own more than 3 linux boxes.
Before I get teared and feathered by linux nerds, listen:
NFS relies on "every user on every box has the same unique numeric user ID. Here the trouble starts: my Debian 4 NAS used, say, dad=301, mom=302, jun=303. Moms laptop with debian 7 used dad=3000, mom=3002, jun=3003. Juniors PC with Ubuntu used dad=1005, mom=1006, jun=1000 IIRC. OSMC and Raspbmc boxes using osmc=1000, dad=1001, … You see? I had to harmonize all these numbers, re-assign ownership of files a.s.o.
At the end of the night, all where identical and NFS worked. …
but now we detected a bug in the xUbuntu login screen. Even after telling it, 300 to 999 are regular users, it denies to offer “dad” and “mom” as login users. we always have to select “others”, then type in dad … what shit.
Finally, CIFS works like a charm with OSMC and the “how much storage left” question is also answered by “tvheadend” plugin which also runs on my basement NAS box. So I had a lot of work but no benefit aside of learning it.
To make a long story short: chose wisely if you really want to use NFS. Ask f.i. Wikipedia about NFS and CIFS. Then make your selection.