In order to start a MiDE session from standard display managers (such as SDDM, included with Debian), you need a XSession file at /usr/share/xsessions/mide.desktop:
[Desktop Entry]
Name=MiDE
Comment=MiDEsktop
Exec=/opt/kde1/bin/startkde
TryExec=/opt/kde1/bin/startkde
Type=Application
X-LightDM-DesktopName=MiDE
DesktopNames=MiDE