If you have an nVidia or ATi/AMD display cards, you should install the display driver after the installation of Ubuntu 11.04. Otherwise, you will be in Classic interface instead of Unity.
Or, may be you can install the 2D Unity instead.
sudo apt-get install unity-2d
Furthermore, the 3D Unity will be installed automatically on Intel display card system.
In addition, if you are going to install Ubuntu 11.04 on a laptop or netbook, you should plug in the power cable; otherwise, your interface after the installation will be in a mess.
If you considered to upgrade from 10.10, I recommended to fresh install as some previous settings of Gnome may affect to the new interface - Unity as well as Firefox 4. Before fresh install, please backup all your data.
By the way, if you encounter the blank screen or black screen on Intel display card system, you can refer to this tutorial.
The following is the video which shows you how to operate with the Unity on my remastered Ubuntu Samiux Remix :
That's all! See you.