1) Under the HVAC tab, when you select Library on the right side of the screen while designing an air loop, you might pick from Coil Heating Electric -> Unit Heater Elec Htg Coil or Coil Heating Gas -> Unit Heater Gas Htg Coil or Coil Heating Water -> HW Htg Coil/Unit Vent Heat Coil. I think that should get you started on the radiators, though I think based on your question, you'll pick a Coil Heating Water item.
2) You will likely generate a plant loop to generate hot water with a boiler and an air loop with the Coil Heating Water item. In the air loop, when you select My Model rather than Library on the right side of the screen, you will see Thermal Zones listed further down. From there you can drag a thermal zone into the air loop and place it left of the Coil Heating Water item.
3) The plant loop must have a Set Point Manager on the outlet side of the supply half. This will set the hot water temperature to, say, 180 oF - there is only one of these. Then, under the Thermal Zones tab, each thermal zone must have a Heating Thermostat Schedule. It is in those schedules where you tell OpenStudio what each zone temperature is supposed to be (zone set point as opposed to loop set point). OpenStudio then will simply vary the hot water flow rate through the Coil Heating Water item to maintain the zone temperature.
