Ok,

Lets see if I can get you running into KDE...

First, I am assume the reason you are running Flux is not due to memory, or being low on it, and the choice as all yours, because from what I hear, KDE can run pretty heavy on the memory, and again, from what I hear, Flux is more "conservative" in the memory usage...

During the "Login" screen, when you boot, or change users, I am going off Knoppix v3.3 here, so it may not look the same, nor may the menu screen be the same, if you are using a different version, but the jest should be the same... In the login screen, mine at least, you select the user you want to login, and then enter the password just below it, on that same screen, do you have a choice of "Window Desktops" ? (it will be a "drop-down" box, and may be showing "Fluxbox", or possibly "Default"....)

If you can locate that Window Desktop box, can you pull that down, and locate "KDE" in it? I had this problem of these "Desktop" environments, the last time I did a "apt-get upgrade", and X was being upgraded, it decided to make itself the "Default", which I have always used "KDE", I had to change the "Default" selection, to force it to run KDE.

Hope this helps,
Ms. Cuddles