Why can't I see computers listed on my DNS server?
The DNS server has to be properly configured by the administrator to have an entry for each service and to accept zone transfers.
I can't see some devices.
The routers on your network must support IP multicasting. The Network Administrator should refer to the router vendor to find out, if she doesn't already know.
I can't see some devices on my older Macs.
Older Macs can see each other. Mac OS 9 Macs can see each other. Mac OS 8.5 and 8.6 clients can see each other because they use version 1.0 of the SLP protocol. Mac OS 9 clients can see other Mac OS 9 clients. Mac OS 9 uses version 1.1 of the SLP protocol.