Originally Posted by Dngrsone
Okay, I tried
sudo apt-get install kde-plasma-desktop
but now I have to choose a default display manager.
I have no idea what the differences are between KDM, LightDM and MDM...
Okay, I guess MDM (Mint Display Manager) is the default.
If not, I guess I will have to learn a few more terminal commands...
No, it doesn't matter which one you picked. Now, when you go to log in, you'll be able to choose which DM you want; by choosing MDM you simply made that the default, i.e., if you don't choose something else, that's what will start up. Once you choose KDM (when logging in), it'll stay KDM for subsequent log ins, until you choose something else.
When you're on the log in screen there's a button/icon/list/something (you'll have to see what it looks like on your distro's log in screen); click on it and you'll see the choices.