I just installed Mandrake 9.1 and when I rebooted it I have a command line for login. How do I go about installing KDE?

Did you install it during the install? It is selected by default so you would have had to uncheck the box.

If you have not logged in at all before, at the command line, enter your username and hit enter, then your password and hit enter. From this point, type 'startx' without the quotes. Mandrake will guide you through a series of prompts at this point, I believe the first of which is to ask which desktop you want as default. If you want KDE, select it here and from that point on, everytime you log in and type startx KDE will start.

Also if mandrake does not ask you if you want X started on boot, after you startx for the first time. You can set it up to start on boot by going into "mandrake control center" you will be prompted for root password then click on "boot" then click on "drakboot" icon. Check "Launch the graphical enviroment when the system starts" if you want it to auto login your user select "yes, I want autologin with this (user, desktop)" select the username you want logged in and the desktop enviroment (you said KDE) you want to come up.

I would also recomend you go to "hardware" and select "display manager chooser" and select another display manager, I have seen problems with the default "MdkKDM" I personaly like "KDM" but the others (not MdkKDM) work nice as well.

I hope this is helpful.