here comes a small suggestion from me.
By default on Mint 13 XFCE there is notify-osd installed for displaying notifications.
On a single screen this might be okay, because the popup is always displayed in the top right corner.
When you have a dual screen setup with left screen as primary the notification will always displayed on the second on the top right..
Even on single screen there is no options to move.
To solve this just remove the notify-osd package (not the plugins!) and install xfce4-notifyd.
xfce4-notifyd responds to notify-send commands out of the box and also will have a little setup utility to move it to differend corners + it will display on primary screen