I recently upgraded from ubuntu 11.10 to the new ubuntu 12.04 . The new 12.04 is cool except maybe what brought you here that is the failure of unity to show you already installed applications. without much ado here is the solution
1) you just need to unistall unity and re-install it again… in the process of reinstalling it it picks some missing packages and that is the fix.
so here is how you go about it:
sudo apt-get remove unity unity-2d-places unity-2d unity-2d-panel unity-2d-spread unity-asset-pool unity-services unity-lens-files unity-lens-music unity-lens-applications gir1.2-unity-4.0 unity-common indicator-sound indicator-power indicator-appmenu libindicator6 indicator-application evolution-indicator indicator-datetime indicator-messages
that unistalls everything
then just do a fresh install like this
sudo apt-get install unity
Fixed and done, just do a log off and come login again or reboot.
Remember to say thanks.