Terminal is an application started like any other. It's in the Utilities tab of applications and is label "X Terminal". Run that ap and a terminal window opens. You're not at what's called the command line. If you enter the name of any application followed by enter key that ap will be started and the terminal will show informational, error or warning messages. You will need to select the running terminal to see those. Ending the terminal application will also close any application started in the terminal. I'm reasonable sure the command you want is mcalendar.