In Ubuntu virtualbox guest, when you try to log-in gnome shell, it falls back to gnome classic. This tutorial will show you how to log in gnome shell session in Ubuntu 12.04 vbox guest.

First of all, enable guest 3D acceleration in Vbox settings window -> Display -> Video tab:

Then start Ubuntu guest and log-in. Navigate to window menu Devices -> Install Guest Additions…

It’ll prompt that could not find guest additions CD image file. Just click “Yes” then click “Download” the image.

You can find the downloading process at the bottom:

Once finish downloading guest additions image, it prompts to mount the iso file:

Click “Mount” then “Run” to run the installer at next dialog:

Give the super user privilege and wait the installer finish:

After that, restart Ubuntu guest and log in with Gnome session.

You’re done, enjoy!