If your NAS device is capable of NDMP, then you can connect the tape devices directly to filer/data mover and perform backup of NAS data directly to tape. Backup Exec NDMP license is required for doing this. Some NAS devices with Windows operating system will allow you to install agents on them, so you can backup the data on those like backing up any other server.
If you do not want to use NDMP, disable it first in NAS device. Create a User defined selection to backup the NAS share. It looks like \\SERVER_NAME\Share. But be ready to see the exception/error "Remote agent is not installed in the server" in job log and this cannot be disabled.
EDIT: Starting from BE 2012, user defined selections option is removed. How to perform share level backups is explained in this TN: http://www.symantec.com/docs/TECH189560
Checke the device and media service account and make a note of the credentials and username.
These credentials should be used in the NAs device to configure the shares.
Once configured this way you should be able to see the NAS and able to back it up
I am trying to backup data on NAS and it has no ndmp capability. I was able to create User Defined selection in BE, choose the appropriate credentials (username and password), and was able to select the folders to backup in NAS to backup. But when I ran the actual backup job itself, it errored out with the message: "e0009b61 - The logon account that was provided does not have valid credentials. Ensure that the user name and password are correct, and then try again."