You question post is bit unclear ." We are using NAS and mapped it to our DLO server. I can't find the mapped drive in the backup selection list."
How are you trying to backup the Dlo server ?If you are using backup exec refer http://www.symantec.com/business/support/index?page=content&id=TECH43305
Dlo server cannot backup itself.
Mapped drives are not shown in the backup selection list
when creating storage location you need to consider the following points
1> You can't map a share as a storage location
2> You can only create a storage location on NAS if NAS has the drive letter
3> If your NAS doesn't proivde drive letter then you have to create the users and allocate them with NUDF. NUDF will work as storage location but you have to do it for all the users as you will not be able to create the storage location if NAS doesn't have drive letter.
Hope this helps