L> I got a little dialog with two text boxes. Only labels: by the top
L> box an icon of a pencil writing on paper. By the lower box a
L> padlock (closed).

the top on is for your username and bottom one for
password but you should also have another icon in the box that gives a
drop-down list when clicked so you can select the desktop you want to
boot into. Select KDE then enter username / password and press Enter.

Next time you boot and want to go into KDE again just give username
and password.

BTW - this entering of Username/Password is well ingrained in the Unix
world which Ubuntu is a child of. If you have only a single desktop
then you can elect to log in automatically but when there's more than
one choice you have this selection / login screen.