How to Model Makeup Air Unit in OpenStudio

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You are correct that the dual duct terminals are not available in OS, but beginning in 1.5 we have the AirTerminalSingleDuctSupplySideMixer. I know you said your model is version 1.4, so upgrading might not be an option for you. If however you want to migrate to 1.5, then to use the SupplySideMixer you add the terminal like any other, then you can drop the FCU on the terminal outlet node. All of the connections will be made for you. You must have the supply side mixer in place before you try to drop the FCU otherwise it will not be allowed inside the AirLoopHVAC topology.

For the dual duct terminal I suggest that you use one of the single duct terminals in OS. Then apply an EnergyPlus Measure to swap out the place holder terminal with the dual duct terminal. Since I expect you only need one leg of the dual duct the connections should all be in place, and the EnergyPlus measure should only need to swap one object for another.

I would recommend upgrading to OS 1.5... it has added a number of features which improve the program, and they've also added a number of additional HVAC objects that weren't in 1.4.

You'll have to build up your AHUs, or start from one of the template HVAC systems. You'll need an OA controller, coils for your heating and cooling, and a fan box, constant or variable, as well as one (or more) setpoint managers to make up the supply side of the AHU. Then on the demand side, you'll assign the thermal zones as well as a VAV box to serve each zone. The splitter/mixer objects are created automatically - they are represented by the branch split, and on the other side where the branches join again.

Again, my suggestion is to try version 1.5, you might find what you are looking for there.