Can anybody exlain:

DNS name or the IP number of an SNMP-managable device.

Is that the gateway ip to the router or does it mean the ip you want mrtg to moniter or what?

Regards

"public" should work, its the broadcast group I believe of the router.

I'm talking about this line:

community@router.abc.xyz

can you explain it?

public@server.ip.address

try it