Installing KDE Software Compilation is not enough to get it run out of the box. A few more steps are required, and we'll try to list them here. If you feel something's missing, contact us with some ideas.
X Window System
Starting KDE Plasma Desktop
After installing KDE workspace (x11/kde4-workspace) either via ports or packages, you can add kdm4_enable="YES" to /etc/rc.conf to boot into KDE Plasma Desktop via KDM, the KDE Display Manager. To get full functionalities, you'll also need to mount procfs.