Need help with Apple Airport Express

Wow. Really? This is one of the very easiest products to setup and have extend your network as you are describing.
But...it sounds like you are wanting this to do two different things.

1) You want to extend your network to your Marantz.
2) You want it to deliver your iTunes content into your Marantz.

Sounds like you have #1 licked now just use the simple iPad app (or iPhone) that let's you configure the AE. Once you see your AE on the app you can edit it to do both extend your entwork and also to act as a receiver for your iTunes library by simply going to the edit button on the top right and then looking down the list for AirPlay and making sure it is enabled then you can name it whatever you like so as not to get it confused with others you may have around your house.

THEN you need to either hookup an optical cable or you can atke that same output into a splitter and fo into an analog input on your receiver. The you go to iTunes and check AirPlay to that device and off you go.

OR...you can get something like Asset UPnP for your Mac or PC and have it access your iTunes library so you can play it through the media player that is in the Marantz.

Once this is setup you will experience very little or NO problems with it but you just need to be sure you have a good solid network and you dot your i's and cross your t's.
This is pretty much networking 101 and I find it odd that so many think of it as black art or something that screws up on a whim.

Do some homework at http://www.computeraudiophile.com/
and you should be in shape in no time at all.

Realhifi, Thanks for your help. I am away for a few days, so I will get back to this when I am home. I'll update the thread at that time. ...Matt

Back again, making progress, getting closer, but not quite there.
I got "AirPlay" enabled in iTunes on the iMac, and on the Airport
Express. I can play the iTunes library thru the Marantz as long as I control
it from the iMac. But, I can't access the library using the Marantz controls
(remote control, or the receiver itself). So, I downloaded Asset UPnP, and
configured it on the iMac.
When I try to access it using the Marantz remote, the following happens:
I select "Network" input then I select "Media Server".
On the receiver display (and the TV) I see "Asset UPnP: matthew's
iMac"
I select this, and I see the usual menu choices (artist, album, title, genre,
etc). I select one of these, and the receiver display says "Please
wait, ..connecting..." After maybe a minute, the TV screen begins
flashing and then it displays the Network Menu again(favorites, Internet
radio, media server, etc). That's as far as it gets. Is this a problem with the
Marantz setup, or the iMac setup..( iTunes or Asset). ?

Update: I think I have it working!!! Thanks to all for your help. A'gon is great!