Google solved the problem. You have to install the package kdm-kde4 to have the greeter plugin configured. I installed this package and now I can lock the screen.
As this is some issue, perhaps add a dependency in kdebase-workspace to kdm-kde4? -- cannot lock KDE 4 desktop https://bugs.launchpad.net/bugs/183800 You received this bug notification because you are a member of Ubuntu Bugs, which is the bug contact for Ubuntu. -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
