From the main menu, choose "Files" and then ".mac".
At least the mu_plus_300_MeV.mac file coming with the application should be proposed. This file contains :
/gun/particle mu+ /gun/energy 300 MeV /run/beamOn 100
You can execute it with the examples/calorimeter. The GUI steps are :
It should start a run by shooting mu+ of 300 MeV in the template calorimeter.