In Software Center some software is not found

For 64 bit Ubuntu systems only, add the i386 architecture before running all the below commands,

sudo dpkg --add-architecture i386

First enable multiverse repository,

sudo add-apt-repository multiverse

Update all the repositories

sudo apt-get update

Install steam package

sudo apt-get install steam

How did i know

$ apt-cache policy steam
N: Unable to locate package steam
$ sudo dpkg --add-architecture i386
$ apt-cache policy steam
steam:i386:
  Installed: (none)
  Candidate: 1:1.0.0.45-1ubuntu1
  Version table:
     1:1.0.0.45-1ubuntu1 0
        500 http://ftp.cuhk.edu.hk/pub/Linux/ubuntu/ trusty/multiverse i386 Packages