KDE Partition manager was removed from the live KDE image long ago due to an incompatibility with udisks2. According to Kevin Kofler, this issue was fixed long ago but the tool was never put back into the image. Consequently, the Plasma Live environment contains no graphical disk partitioning tool at all.
Added (back) to kde-desktop comps group: https://git.fedorahosted.org/cgit/comps.git/commit/?id=af1724b305c3665658a6e417f187c9055cae3c05