If you try to use X-apps as root you probably get:
root@mrk-fujibox:~# kedit
kedit: cannot connect to X server :0.0
There are several fixes. I usually make a .profile in roots home dir with the following content:
root@mrk-fujibox:~# cat .profile
export DISPLAY=:0.0
You can check the variable with:
markus@mrk-fujibox:~$ echo $DISPLAY
:0.0
To create the file you need to use a console editor like mcedit:
markus@mrk-fujibox:~$ su
Password:
root@mrk-fujibox:/home/markus# cd
root@mrk-fujibox:~# mcedit .profile
-use F2 to save and F10 to quit, tips are at the bottom of the screen.

If you try to login to KDE as root it won't work. You should never login to KDE as root as it's a security risk.
To do root stuff you can use:
- sudo
- su
- sux
- kdesu kedit (or some other app)
If you really, really need to login to KDE as root you can search this forum for the answer, it's here but I'm not posting it in this thread for the simple reason that it shoudn't be used.