Spent yesterday afternoon updating 6 TIUs and 5 remotes for myself and a friend, to DCS v6.10. Having learned to deal with the seemingly random "stuff" that computers dish out while we're trying to get something done (necessary to this and many other processes), thought my experience might be useful to others who have put off dealing with this for years like me :-) This is all from working on a Windows 10 PC, no Mac experience to offer. (Barry's WiFi DCS Companion book, 2nd edition, has a nice long paragraph about what works if you're trying to use a Mac, in the section there about software updates. It has more information than my "regular" DCS Companion 3rd edition on that topic, for some reason.)
One discovery was that only the DCS Loader software has to be "opened and installed" after you download and unzip it. The DCS 6.10 TIU and Remote software just has to be unzipped to whatever folder you want it in, and the Loader software finds it and uses it "as is." The Loader actually finds the correct file on its own when you click START in the dialog box (see below), so it brings up the TIU software file in a window showing it in the folder where you put it when a TIU is connected, and the Remote software file when a remote is connected for update, making it easy to choose the right file and "open" it to trigger the update.
Mine are Rev L TIUs, so just the type B (printer-style, at least for my HP printers) USB cable and the 1/8" (3 mm) stereo mini jumper cable are needed for the TIUs, and the 4 conductor phone handset cable for the remotes. For my friend's TIUs, older models, an adapter from the USB "B" end (printer end, to me) to 9-pin serial cable at the TIU, is needed, plus an MTH auxiliary power supply to power the TIU. Pay attention to the male-female issue and make sure the serial cable adapter has the right "end" to match what the TIU has. I discovered my friend's old adapter cable was obsolete; it would not work, and finally my Windows 10 computer gave me an error message saying it was not compatible, or similar words indicating it no longer would work with current hardware. But before that I got nonsense error messages that gave no clue that the cable was the problem, so persistence in this case finally yielded up the "right answer", when the computer decided to bestow it on me! Also, a TIU with version "2.xx" early software needs a "Factory Reset" before updating: Connect remote with phone cable to Remote (not AIU) port on TIU, turn on remote, MENU>SYSTEM>TIU option>TIU RESET>FACTORY RESET. This does erase the data on the TIU, no getting around that. The one old TIU of my friend's that got this treatment, then appeared to accept the update (success noted on computer when done), but when trying to confirm the new version with remote connected, would not give a version number the first couple tries, and then just said "TIU ERROR" on the remote screen. We decided that TIU must be Tango Uniform and is headed for the electronics recycler.
Fortunately since I never throw anything away, I have an old male to female serial cable, and a short USB "B" to serial adapter bought for possible TIU use, that went together and did work to update his TIUs.
The remote updates went swimmingly, the only big deal there is making sure not to plug the phone cable into the TIU's AIU input because that can lead to damage--it's the same type connection as the Remote input.
MTH's DCS Loader 5.0 dialog box instructions are now entirely accurate, and following them works 100%. However, that computer randomness does bite when it feels like it. A bunch of times, when I had the cables hooked up for a TIU update, and clicked on the START button in the dialog box, my PC gave me error messages saying it couldn't search for the device on one Com port after another (Com4, Com5, Com9, etc etc) and then others implying the device didn't exist so it couldn't find it. The only solution there was I unplugged and reconnected, after 10 sec or so, the USB cable from the computer, and tried again. This does not require using the "remove hardware safely" function on the computer, there is no concern about data corruption etc from just unplugging the cable when the update is not actually underway. I did close the Loader program while doing this, and reopen it after the USB was reconnected. Sometimes the junk messages happened the next time and the next, but eventually the process just went ahead and worked. Dumb persistence was the main requirement.
The net of all this, is it took a full afternoon of putzing to get 6 TIUs and 5 remotes updated, almost all of that time used for the TIUs, because of all the little stuff that stops you and takes you sideways for a bit, before you can resume forward progress. However, have to say it never got to the head-banging stage this time, unlike doing the same tasks for earlier software updates. The bugs have been worked out of this process by MTH over the past several years, it seems. The head banging, if you have any, will likely be just ordinary computer "stuff" that happens in lots of situations. If you've been putting off this work, and have beautiful PS2 or PS3 locomotives occupying shelves or boxes instead of pulling trains, I encourage you to screw up your courage and give it a try now 
Hope I've remembered to include all the parts that matter here. Good luck, Don
