Something somewhat similar to the Kubuntu 14.04 problem - a few months
ago I did a fresh install of Mint (Debian Mate) in order to have its
wifi functionality.  Then installed TDE base and several useful
packages, which work.  Later I installed TDM and other packages it
seemed to need and chose TDM as the login mechanism.  During login
using TDM, the list of available sessions doesn't include TDE, and the
subsequent desktop is still Mint.  So maybe I missed some packages or
configuration?

Robert