if you want to install it when you make a fresh install with obarun-install, pick the number 2 and choose plasma on the list displayed. Note, for the moment plasma is installed with runit.
if you want to udpate an installation to use plasma, install this following package :
$ sudo pacman -S plasma-meta kdebase-lib
if you use .xinitrc to launch the X server, change it like this:
# !/bin/bash
exec startkde
and erase/comment all other line in this file.
hope this help
if you have any other questions about it, don't hesitate