root user, graphical programs

[Pieter Donche]

FreeBSD 7, KDE 3.5
To install Matlab (in linux compat mode), one must execute the
matlab install program as root. The installer is graphical.
When from a KDE terminal window, I switch to root (# su -)
and try a graphical program, e.g.  # xpdf, I get Can't open display.
The FreeBSD handbook (10.5.1) says in that case type setenv HOME ~USER
(USER the username where the su command was given). This doesn't help
I still get Can't open display.

What's wrong and how to solve it?