A training device on MyFlightbook is just another aircraft in which you can log flights. The difference is that the system knows that it isn't an actual flying machine. When you add an aircraft to your account, you can indicate that it is a real aircraft or one of several kinds of training devices (discussed below).
If you are adding a training device to your account, you might choose to create it using the model of the training device (e.g., Frasca, Redbird, Flight Safety, etc.), or you might choose to create it using the model of aircraft being emulated (e.g., Cessna 172, Boeing 737, etc.) Usually, this is just a matter of personal preference, but in the scenarios where you can count your sim time as "time in type", I would recommend doing the latter.
When you add a training device to your aircraft list, it is auto-assigned a pseudo-tailnumber that begins with the prefix "SIM". Partly, this is a mnemonic, but the bigger benefit is that since no country has a naked "S" or an "SI" prefix, so there's no opportunity for a SIMxxxx tailnumber to overlap with an actual flying machine. The remainder of the tailnumber will reflect the model of the "aircraft" that was selected. E.g., a Boeing 737 simulator might get SIMB7378 or similar.
To log a flight in a training device, simply choose the SIMxxxx aircraft from your aircraft list. Log the time of your session in the "Ground Sim field. Most of the time, you'll leave the PIC, SIC, and Total Time fields blank (zero), but if your training session is done in appropriately rated sim and in accordance with regulations that allow you to log it as time-in-type, then you may be able to fill in these fields. I leave that judgment to you (consult your instructor!), but be aware that if you do log time in the total time field, it will add to your time-in-type. E.g., if it's a multi-engine sim and you log 1.5 hours of total time, that will add 1.5 hours to your total MEL time. So don't log time in that field if you don't want it to accrue to your totals.
You generally also want to log the serial number of the training device. Throwing that into the comments for the flight is absolutely fine. But if you wish to keep this orderly, there is also a property titled "Simulator/Training Device Identifier" that you can employ.
While there are all sorts of certifications for training devices, MyFlightbook really only cares about how they should be treated when computing things like passenger-carrying or instrument currency (61.57(a), 61.57(b), and 61.57(c)). As such, training devices are broken out into four buckets on MyFlightbook:
- Uncertified. Think Microsoft Flight Simulator. Any approaches or landings are ignored when computing 61.57 currency.
- ATD. This has a formal definition from the FAA. If you log a landing, it will be ignored for 61.57(a)/(b). But instrument approaches and holds apply towards 61.57(c)(3) and 61.57(c)(4) (more on this below). Note that 61.57(c)(3) requires additional maneuvers beyond just approaches and holds.
- FTD or Simulator - IFR approaches. You know, I've never quite followed the formal distinction between an FTD and a Simulator, but while one exists, it doesn't matter to MyFlightbook. Like an ATD, landings are ignored, but approaches and holds do count towards instrument currency, specifically 61.57(c)(2) and 61.57(c)(4).
- FFS or Simulator - approaches and landings. This is exactly like the FTD/Simulator above, except that if you log landings, they will count towards 61.57(a) and 61.57(b). This almost always requires a full motion "Level D" certification.
Sims are mostly used for instrument training, and pilots will often pad out their instrument currency with sim time. But be careful! A few years ago, the FAA rewrote 61.57(c) with the goal of making mix-and-match of real aircraft and training devices easier, but they screwed it up and actually wrote words that make it harder. See the currency notes on the MyFlightbook site for the gory details, but the short summary is that you can't mix and match approaches/holds in a real aircraft with those in a training device. My understanding is that the FAA is working on a rewrite to fix this, but we will have to wait and see.
UPDATE: As of June 2018, the FAA has released an update to 61.57(c), and as of Nov 26 2018, you can mix-and-match aircraft and FTD/ATD/FFS all you like, getting the full 6 months in each. MyFlightbook will automatically cut over to the new rules when they take effect.