I have the same problem. I'm using the latest official viewer. A few days ago, my received items folder vanished. In hindsight and after looking at JIRAs, I suspect opening a secondary inventory window might have triggered it, but I cannot say for sure.
The point is that it vanished, and when I tried to force it to appear by buying a demo from the Marketplace, the button appeared, but its contents are just a repetition of my normal inventory, not the Marketplace items.
I cleared cache and restarted. Didn't help. I re-downloaded the latest viewer, did a clean re-installation, didn't help.
I don't really know what to try next. Guess I'll download Firestorm and see if the setting to make it appear as a system folder will make me able to get my Marketplace purchases, but I really actually -like- the official viewer and would like to continue using it.