Need help with Apple Airport Express

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!