private message popup

i do not know how to change codes but i want to have a message come up for new private message.
i went to display , posts and private messages, privmsgs_popup then edit but i have no clue how to enable a popup

It is part of members' profiles:
Profile > Preferences
Display a pop-up window for new private messages : [] Yes [] No
Some templates may open a new window to inform you when new private messages arrive. Notice : Make sure your browser's popup blocker doesn't block popups of this site.

thanks somuch