An external model can be run by creating a new launch configuration. To create a launch configuration:
Go to Run -> Run Configurations.. to run the model or Run -> Debug Configurations.. to start a debug simulation.
Right click on Poosl Simulation and select New to create a new launch configuration
Click the browse button to open the file dialog. In the dialog select your file and click open.
Click Run/Debug to start the simulation.
More information about launch configurations can be found here.