1.
Settings - General - More settings - SAMBA service - Start.
This will enable the Samba server and share all attached storage.
2.
Several ways. The NAS could be setup to share using Samba, NFS or uPnP/DLNA. Samba or NFS are best.
Once the NAS is setup go Video - Libraries path management. (If you don't have a library set up already this will appear automatically, if you do then left remote key - settings - libraries path management. Then 'Add network path' - automatic search / add samba server / add nfs server.