Open your personal.rc. There'll be a few lines starting with "*desktop" which are read by desktop2 and some lines starting with "jdesk" for themes using jdesk.dll. The desktop2 documentation is here http://lsdocs.shellfront.org/desktop.php - what you want to add is something like
*Desktop lbutton !PopupTasks
!PopupTasks is a !bang registered by popup2, as you can find out here http://lsdocs.shellfront.org/popup.php
For the appropriate jdesk lines you'll have to check the readme in the jdesk zip as I'm not familiar with its syntax.