Stephen,
Make sure you have DNS configured correctly on the box running EE.
The software does a DNS reverse lookup on the IP addresses found
in the routers arp table.
Firstly, you need to make sure you can do a nslookup on the DNS server.
Secondly there must be a DNS server which is authoritative for this IP reverse zone you are trying to resolve, with entries for all the IP's you have found with the mapper.
If the DNS or DHCP server is Win2k, and the workstations have DDNS enabled, then these entries should be there.
Try this on the EE pc/server from a command prompt.
ping -a xxx.xxx.xxx.xxx
If it can resolve the name of the IP then you should be set.
-=Cheers=-
NG