OK, so that's pretty easy. I assume either your Extreme or your cable modem is handing out IP addresses via DHCP?
If so, then you just need to set the Express to either "Join a Wireless Network" or "Extend a Wireless Network," put in your creds for the Extreme network, and you should be good to go. I would recommend setting a static address outside your DHCP range on the Express if you're doing the Join option.
As for the Marantz, I can't really help you there.
I didn't do it, no one saw me, you can't prove anything.