I have had to autodiscover a networks without SNMP in Visio.  It's a pain but can be done.  You have to play a bit with the settings but definately do the "discover SNMP and non-SNMP devices", "ignore previously discovered", and just ignore the community strings.  One thing that does make this difficult is if you don't have a default gateway or a router to add....this is hell without one.  One huge problem i've had is with my default gateway.  Like I said without a router or default gateway your pretty much dead in the water.  But you can still do "some" discovery by going on and designating specific IP addresses.  Other than that it's worthless.  There is another tool that does work regardless of SNMP, Routers, Default Gateways, or anything else.  It's by Visionael.  You can download a trial from their site for discovery only (no diagramming).  But I do know it works.  No idea on the costs though.  Hope this helps.