Even after setting the universe settings from the ubuntu software sources, it always gave me a message saying ibus-m17n package is unable to locate.
Then I found this thread (http://askubuntu.com/questions/105544/install-sinhala-unicode-pack) from the ask ubuntu site and there was a answer given by Maduka Jayalath saying ibus is the solution.
So I followed the link (https://help.ubuntu.com/community/ibus) and installed ibus-m17n separately.
sudo apt-get install ibus-m17-n
Finally I executed the instructions given by Siyabas site again and it successfully enabled Sinhala unicode Wijesekara layout in my Ubuntu 12.04 LTS