I have installed Elementary OS's Pantheon on Ubuntu 14.04 following the instructions in here installation guide.
After doing what the first answer says, I rebooted and found that there is no icon to choose a session on the login page. Upon logging onto Ubuntu, however, I notice that there exists the Elementary theme and a few other things so it's not like nothing was installed... But still I cannot access it.
I then followed the instructions on the second answer, but once again it was to no avail. No way to choose another session.
What can I do?