Ok, so your network configuration, with the AirPort Express Base Station (AX) is as follows:
ADSL modem > (Ethernet cable) > [Ethernet port] AX > (wireless) > iBook G4...correct?
If true, you will need to set up the AX first. I suggest you connect your iBook to it directly, using an Ethernet cable. (It's okay to disconnect it from the modem.) Then, using the AirPort Admin Utility, make the following settings:
AirPort tab
- Base Station Name: <anything you like, or leave the default>
- AirPort Network Name: <anything you like, or leave the default>
- Create a closed network (unchecked, of now)
- Channel: Automatic
- Mode: 802.11b/g
Change Wireless Security
- Wireless Security: Not enabled (for now)
Note: The next section will depend on, whether or not, your ISP requires PPPoE to connect to their ADSL service. If it does:
Internet Tab
- Connect Using: PPP over Ethernet (PPPoE)
- Account Name: <your ISP account name>
- WAN Ethernet Port: Automatic (Default)
...and if it doesn't:
- Connect Using: Ethernet
- Configure: Using DHCP
- WAN Ethernet Port: Automatic (Default)
Network tab
- Distribute IP addresses (checked)
- Share a single IP address (using DHCP and NAT) (selected)
On your Mac:
System Preferences > Network > AirPort > TCP/IP
- Configure IPv4: Using DHCP
