Before you can download the free version of AVG, you need to register with Grisoft...unless of course you downloaded the software from someplace like Majorgeeks, in which case you're not sent a serial number that you can use to install the program. When you register with Grisoft, you're sent a serial number to your email addy...which you would need to enter in the Serial Number field, in order to progress with the install of the program.
Try downloading AVG 6 directly from Grisoft...
http://free.grisoft.com/freeweb.php