Did a "cmake -DCMAKE_INSTALL_PREFIX=/usr && make" and used checkinstall to build a deb package. Installed fine and I can add it to my desktop via the Add Widget Dialog (it's in the list of installed widgets: konsole terminal session).
You don't need to add it through the install new widget dialog if you execute a make install or checkinstall after cmake.
Have another problem with it:
It only adds a plasmoid container without content - no konsole in it. Installed is /usr/lib/kde4/plasma_applet_konsole.so and /usr/share/kde4/services/plasma-konsole-default.desktop.
EDIT: Found my error - oh boy...
The package by default is named konsole and that overwrote my KDE konsole. Of course the widget window is empty... Reinstalled KDE's konsole and rebuilt the plasma-widget-package and gave it another name - now it works! Alas you cannot make that widget semi-transparent, *that* would be cool.