When you get to the "Installation Type" page, try selecting "Something Else" instead.
Then, go to "Manual Partitioning". From that point, create a single 1GB partition (unencrypted) for the "/boot" filesystem, and then use the rest of the drive as an encrypted volume. It will ask for a password, and then you will see a new encrypted drive appear. Use that newly created encrypted drive as the "/" filesystem.
I just recently installed a system this way, and it worked well in Ubuntu. I couldn't do it in Kubuntu though, I don't think the Kubuntu installer supports encryption.