Sorry, I do not use MS Outlook but it seems you got it.
When you have Outlook opened, how does the plugin appears on avast (left click the ‘a’ blue icon on the system tray), can you see if it is running?
After you did the step above, can you see if the plugin configuration is ok, I mean, scanning both inbound and outbound mail?
Don’t you see the header (for instance):
X-Antivirus: avast! (VPS 0432-2, 04/08/2004), Inbound message
X-Antivirus-Status: Clean