There is no specific way to activate that icon, the display of the icon/s is the same and it should be none or both displayed (Program Settings, Appearance, Show avast tray icon) . So there is some thing wrong with your installation if only one is displayed.
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
If that doesn't work try, uninstall, reboot, install, reboot.
I think you would be better served by having the icons merged, the VRDB only runs every three weeks so I feel it is a bit of a waste to have it sat in the system tray. You could edit the avast4.ini file (notepad or any text editor), [VRDB] section and add the line MergeIcons=1 if it doesn't exist or change the value to =1 if present. This will fource the merge allowing access to the VRDB actions from the 'a' icon.
[VRDB]
MergeIcons=1