Instructions
Turn on your Xbox 360. If Xexmenu does not automatically load (which it may not, depending on your current system configuration), select "Game Library" and then the icon for "Xexmenu" to launch the Xexmenu software.
Scroll through the list of content on Xexmenu until you find your Xbox Live Arcade menu option.
Select the Xbox Live Arcade menu option to launch the service from within the Xexmenu software.