Expanding AirPort Express network with ethernet adapter

What you need to do is to configure the "ZyXEL ZyAir B-400 Wireless Lan Ethernet Adapter" to join the wireless network created by your Airport Express. Typically the configuration process for these devices involves specifying the SSID of the network it is to join (ie the Airport name of your Airport Express), the type of wireless security protocol your Airport Express is using (ie WEP or WPA), and the wireless network password (ie WEP key or WPA password) required to join the Airport Express wireless network. Doing all that is sometimes easier said than done - you need to follow the instructions provided in the user manual for the device. I note that you said "I can already see my AX network from the bondi" - what exactly did you mean by that?

As for the iMac's configuration - all you need to do is configure it to obtain an IP address using DHCP in System Preferences->Network->Show ethernet built in->TCP/IP tab.

Once configured in this manner (a) the iMac should be able to access the internet and (b) you can enable file sharing on the two Macs to exchange files between them via their common wireless connection to the Airport Express.