At the moment the installer doesn't support installation on lvm or raid volumes. There's an anouncment
for an new version of LMDE with an improved installer, which should be released this month. Maybe it also supports that kind of installation.
Nevertheless it's possible to get a running LUKS setup, although it requires some effort. Basically you install LMDE the normal way on a seperate drive/partition/computer and then copy all the files to the encrypted drive, you have created using the manual mentioned. Afterwards you have to install grub, cryptsetup, lvm and update your initramfs.