If u use DHCP u usually shouldn't set the IP etc manually as that's what DHCP is supposed to do for u. If not, the gateway IP is usually that of ur router, and it could be also ur best choice of DNS at this stage. As long as ur dealing with IP addresses the dns doesn't really matter.
If u've set the gateway address manually and not pointing at ur gateway it could cause your dm500 to be sending its responses to the wrong place, ie no response on ur mac or pc. BTW mac or pc doesn't matter as far as IP goes.
If u are able to telnet or get a terminal session with ur dm, try pinging from it to an active IP address on ur network to see if that works.
Also, in the config of most consumer routers/hubs there might be a ping tool, and the router should also be able to give u information on the IP address given to ur dm500 by DHCP.
good luck