I very recently went through the process you may be about to experience. To get the few (but valued) MTH engines operating to their full capabilities on my TMCC layout I had to add the MTH TIU, an MTH Remote, an MTH Z500 transformer (without the Z500 Controller) to provide an aux power input to the TIU, and the additional cables SirCaptain mentioned above to successfully interface the MTH devices to my TMCC elements.
Given my decision to operate the TIU in the passive mode (as my Lionel transformers/powerhouses/powermasters/etc already controlled my layout), I used the Z500 MTH transformer plugged into the TIU's Aux Input to simply power the TIU. In the passive mode the TIU is sufficiently powered to receive and send commands, but not forward voltage to the track(s) to operate engines.
Among the various interfaces on the TIU there are 4 (track voltage/signal) outputs....two are for fixed voltage and two for variable voltage (this might make more sense after reading the TIU user manual). Given what I was trying to do with my specific TMCC layout (plus information found in the TIU user manual and forum discussions), I was led to believe I needed 4 fixed voltage output TIU channels ......which a single TIU is capable of providing....1 fixed TIU output for each TMCC track block I planned to connect to. The TIU firmware/software allows for variable channels to be reconfigured as fixed outputs and the Remote can be used to do this ( directions for this are in the TIU or Remote user manual).
The wiring is key.....in my case, I wired each TIU output's pos & neg terminals in a star pattern to one of the four TMCC track/power blocks already in use on my layout.....TIU output 1 to two places on layout block 1, TIU output 2 to two places on layout block 2, etc. My layout actually had 6 intended track blocks (or power districts) and interfacing the MTH TIU to the track exposed some existing, unintentional "cross-block" wiring I had in my layout which prevented the DCS from working. It had been inconsequential to TMCC operations as I used the trains. However, I found power feed (i.e. center rail) isolation of each of the blocks was key in getting MTH DCS operations working correctly.
You'll also need to make sure the software/firmware versions of each of the MTH devices (TIU, Remote, other?) are the latest. MTH has the software/firmware freely available on their website and instructions can be found in the manuals and online.
One additional item to consider procuring is Barry's book on DCS.
As it stands today, I can easily operate my MTH engines using the MTH DCS Remote and my TMCC/Conventional engines with my Lionel remotes, but both remotes are required to access full respective capabilities. I subsequently added the MTH WIU (yet another experience) and am enjoying being able to operate the MTH engines through the app on an iOS phone.
I hope this helps.