Hi, I seem to be having the same or similar problem. I have been a Minty user since Helena, but this has been the most difficult installation to get right. I am running an ASUS A6000km laptop, with Nvidia Ge7300 graphic etc, the live cd installation was fine and showed the Gnome 3 and menus etc, plus the ability to swap to MGSE ok. So I installed in my usual way, retaining the home folder and formatting the previous minty installation. All seemed to go well, and I installed the DVD meta package and all the current updates. The installation only seems to give me options of Gnome, Gnome Classic, Gnome Classic ( without ...) etc. Not MGSE or Gnome 3 options. I did not install the Mate package but I do have the mint-meta-mgse package installed. I cannot get the screens that I had on the live CD, no infinity icon in the top bar or menu down the bottom bar, so I don't think that I am running Gnome 3. I log out and try to select something different, but I only have the choices above. I tried removing the Nvidia driver, but it made no difference, so I re-activated the recommended driver again.
Everything seems to work ok in the classic menu, but I only moved to Mint 12 to get the new shells. Any ideas please?