Kontact plugins are KParts that get loaded at runtime. To integrate your KPart into kontact, you will need to have a .desktop file like this:

ls /home/kde-devel/kde/share/kde4/services/kontact/
akregatorplugin.desktop     knotesplugin.desktop
akregatorplugin3.2.desktop  korganizerplugin.desktop
journalplugin.desktop       newstickerplugin.desktop
kaddressbookplugin.desktop  plannerplugin.desktop
karmplugin.desktop          specialdatesplugin.desktop
kmailplugin.desktop         summaryplugin.desktop
kmobiletools.desktop        todoplugin.desktop
knodeplugin.desktop         weatherplugin.desktop

Writing a KPart

The following is what you do

write a subclass mypart of KParts::Part
- use a Widget in it and use setWidget to get your widget into the KPart
write main.cpp
- create a KMainWindow* mw
- use new mypart(mw) to get your part into your main window