I don't think the problem is a DNS problem. You have the names in other parts of the system. There are two things you can do.
1) From the map view, go to EDIT -> MAP PREFERENCES. Then go to FALLBACK RULE. This is a menu that allows you to set the order in which device labels appear in network topology views. If you want to change the order permanently, make sure you EDIT DEFAULTS, pick the order you want, and check APPLY, OK, and then OK again.
2) If you ever want to change the view to something else while your looking at the map, (ie.IP to DNS Name), from the map go to VIEW -> DISPLAY LABELS, and then pick the display you want to see. (IP, DNS or Sysname). You can also CLEAR LABELS from here which lets you look at an uncluttered map with NO labels. Hope this helps.