How do I change the IP address that the Admin Server listens on in iPlanet?

The IP address is actually stored in iPlanet’s directory under the server connection object. In this case the dn of that object is:
cn=configuration,cn=admin-serv-win2k,cn=Netscape Administration Server, cn=Server Group,cn=win2k.delaware.dirwiz.com,ou=delaware.dirwiz.com,o=NetscapeRoot

The attribute nsserveraddress contains the IP address - you need to use a utility like LDAPmodify to change this address. To allow the Admin Server to listen on all IP Addresses, you could remove the attribute entirely.