Since you are an experienced user I'll skip the usual "what is your IP address ?" routine and give you two of my hints.
First, the SLC-5/05 has only recently come to support the EtherNet/IP browse response. Controllers that are more than two years old will probably not have the firmware that allows them to be "seen" by the RSLinx EtherNet/IP browsing driver. They will still be accessible, as are all PLC-5E controllers, via the Station/IP correlation list in the regular Ethernet Devices driver.
Second, the EtherNet/IP browse can be blocked by several common firewall and VPN products. The Windows XP SP2 firewall, the default settings of ZoneAlarm, and CheckPoint VPN software have all been the culprits on various installations that I have seen.