Really depends on your engines. If you have a regular TMCC engine you can't use it when its set to Legacy. If you have a Legacy engine and want to run it as TMCC (Only reason I can think is to MU it with an older engine) You set the engine ID, press the info button,m press the Scroll button until the Control tab is selected . Then rotate the wheel until TMCC is selected and and press the button below it
But why?