To celebrate Steam for mac released
,Valve decided let gamers download Portal
with free before 24/5/2010.First go to stream website
and download stream installer
.Create your Stream account using stream installer ( if you don’t have any stream account ),click ‘click here to get start’ in spotlight after log in,then click ‘get portal now’ to let portal game register in your stream account,
Now you can download Portal game into your PC by click ‘INSTALL GAME’.