If the NAS can do iSCSI it's a lot easier. Set up iSCSI on the NAS, set up iSCSI on the Windows server and then present the LUNs to the media server.
If it can't do iSCSI, then you would set up a B2D location as \\IP_Address\Share (after creating the share), and provide the user name and password. Most times you have to add the BESA as a user on the NAS in order to authenticate.
Then simply arget your jobs.
Just to be clear it would help if we knew if you are backing up to this share or trying to backup from this share.
CraigV's answer is about backing up to the share as a storage target to hold the backups, Marianne's answer is from the share where the data to be backed up is held. (Hence two different perfectly valid answers based on the info you gave us that seem to be completely different in what they are describing)