AST doesn't use the mic on the BL5. You must have a BeoSystem 3 mkII with the latest s/w to use AST, or upgrade (hardware module) the previous BeoSystem 3 mkI. Most BeoSystem 3 mkII's distributed after January 2010 should be ready for AST out-of-the-box.
You simply set two points along two of your walls, A and B, as described in the setup manual: A and B must be at least 2 meter appart, and all speakers must be visible from A and B. It is fairly clever and very impressive. You can have a default video focal point, and a different default audio focal point - worth the AST invesment alone IMHO. And you can assign listening positions such as Dining, Cleaning, Reading, Party etc.
What you do need is more than 3 speakers and a Beo5 (you assign names to the listening positions, these can not be selected from a Beo4).
Roger