Here's just an update to this issue, though I see no useful information has come along. I do realize (and did when I bought the device) that it was no longer supported. The firmware updates do give some strong indication of Rhapsody support, though.
Long-story-short, no beans. But maybe this account will be helpful to future users, or me if anyone figures this out.
Thought that perhaps Rhapsody would appear if I could roll back to the 1.01.16 firmware (I had upgraded direct from pre-installed firmware to 1.02 -- though perhaps that was my problem). Rolling back the firmware on the DSM-750 isn't easy. I thought perhaps a full system reset would do the trick, but no dice: It will reset all Media Center and network settings, but will not roll back the installed version of the firmware. Inserting a USB stick with the 1.01 firmware files also won't work: system won't recognize an older version of the firmware. (The two versions of the firmware are available here, on this D-Link page -- you must click the support link for "United States" to see the firmware downloads:
http://www.dlink.com/products/default.aspx?pid=DSM-750&tab=3)
UNLESS (and I saw this posted, unresolved, on another thread) you do a very simple thing and manually change the .txt file that is part of the firmware update. Just change the first string of the text file, which identifies the version, to anything higher than 1.02.09 (think I used 1.02.15 or something) and it will perform the "upgrade," which is really a rollback to 1.01.16. Don't update the filename it points to, of course -- just the first reference in the txt file.
Figured I'd do this, see Rhapsody, then just roll back up to 1.02. But Rhapsody didn't appear with 1.01.16. Still a bit mystified at that, since it would seem as if these firmware updates were locked in stone with the lack of continued support. But that's about the end of my ideas. Upgraded again to 1.02 (from 1.16) and just kind of glad I didn't brick the unit with the rollback!
Happy to hear if anyone has fought through this more successfully, or can confirm why Rhapsody (and, actually, some of the other online services supposedly included in these upgrades) don't appear. Guess I can deal without Rhapsody in the upstairs bedroom! (And actually Live365, the one online service that appears, is fully functional if you go to their web site and set up an account to add presets -- most stations you'd surf for via the DSM750 are dead). But still curious at this point.
Another update: Installed PlayOn. Works fine with MediaLounge. But not worth the price just for Netflix.
I'll just add that I continue to be amazed and very pleased with the streaming abilities of the DSM-750. HD TV shows move from my wireless router downstairs up to my bedroom extender instantly, without a glitch or a hitch. Even fast-forwarding and rewind are flawless. Pretty cool.