root by default is not allowed to access the users display; either run it as a normal user or yield access to the display.
You can run programs form a normal user as root by using the run dialog or using the kdesu program, it generates the same dialog that PBIs use.
user-$ kdesu fooapp
You can also run xhost to give root access to your users display (xhost +root I think) and then su
to root, and set the DISPLAY variable. If you do echo $DISPLAY before the su, you'll see the value.