MRTG works just fine with recent versions of the Sonicwall OS, but I had
trouble finding documenation on enabling SNMP on a Sonicwall PRO 1260
Enhanced.

Ultimately I found the following solution on the sonicwall user's forum.

SNMP must be enabled in two different places.  From the web interface, the
first is under System->Administration->Enable SNMP.  The second
is under Network->Interfaces-><interface you want to access SNMP from, (LAN
in my case)>->Configure->Management->SNMP

*Note* The interface on which you enable SNMP is the interface from which you
want to access SNMP, not the interfaces for which you want SNMP information.