Backing up an EMC NAS

We need to backup some files and folders on an EMC NAS. We are able to map the drive on the windows machine and access the files to it but it cannot be found in ABR 11. We cannot use the UNC or network paths as ABR11 will continue to search for the network path and eventually not find anything. We have also tried to use the drive letter manually input into the UNC dialog box but we are prompted for credentials which we have provided but the prompt keeps coming back. We are able to ping the NAS we are trying to backup.