I was using the KDE 64bit flavor when I ran the installer. I was also starting with a completely blank drive, no MBR, no partition table, and used gparted in the installer to make the partitions. After exiting gparted, and the installer continuing, it would not show me any of the partitions I just made to use as the root partition. The only thing on the dropdown menu was the nbd mount (as I was PXE booting). I closed the installer, and re-ran it, and it still wouldn't show anything, until I went to the first page and hit the "clear configuration" button. Then when I proceeded, all the partitions appeared in the dropdown to be selected as the root partition.
Edited to add (after looking at the screen captures in the manual Devil mentioned above), I'm sure I clicked the refresh button, and still nothing showed up. Does the refresh button in the lower box even have anything to do with actually choosing the root partition in the upper box? If so, that seems like a bit of bad UI design to me. I need to stick another new drive in and install again anyways, so I'll pay more attention to exactly what I have to do to get the partitions to show up, and be sure to try _all_ the refresh buttons.