what version of ubuntu did you install ? desktop is the one version of ubuntu which support unity as desktop manager , because ubuntu server edition start with a shell without GUI optimize for this case.
How to get it ?
Ubuntu server Edition
From Ubuntu Docs (https://help.ubuntu.com/community/)
What's the difference between desktop and server?
The first difference is in the CD contents. The "Server" CD avoids
including what Ubuntu considers desktop packages (packages like X)
but includes server related packages (Apache2, Bind9 and so on). Using
a Desktop CD with a minimal installation and installing, for example,
apache2 from the network, one can obtain the exact same result that
can be obtained by inserting the Server CD and installing apache2 from
The Ubuntu Server Edition installation process is slightly different
then the Desktop Edition. Since by default Ubuntu Server doesn't have
a GUI the process is menu driven, very similar to the Alternate CD
installation process. Ubuntu server install by default a server
optimized kernel. Ubuntu Desktop will receive a 3 years support,
Ubuntu Server will be supported for 5 years.
How to Remote Administration (like putty, remmina etc...)?
ubuntu server recommend this two package :
Chapter 5. Remote Administration
P.S: If you have got a desktop installation you can see this post, the first option doesn't work because there is a bug:
How do I boot into console mode?