The current version of June 14th, 2010, is 2.0.7 build 271. Please download the iRed2.dmg package from here (about 7.6 MB). You may have a look at the Release Notes.
Depending on your browsers settings, the iRed2.dmg package will open automatically or by double-click. You'll get a new Finder window containing the application:

Just drag-and-drop the application icon to your Applications folder or anywhere you like.
All updates of iRed2 will be announced by the built-in Software Update mechanism (Sparkle, by Andy Matuschak), until you have switched automatic version checking off in iRed2's preferences.