I don't know about the NAS-XSR but I do the same for DSS and posted my config some days ago, maybe it helps.

The main problem you might have (now speaking only about my DSS):
DSS supports snmp v2c but no 64Bit-counters (ifHCInOctets/ifHCoutOctets). The 32-bit if*Octets counters wrap at 114Mbit during the 5-minute polling period of mrtg..

Michael