[Dennis Allen]"How do I engage the trim option and the EXT. Channel select?"
In the camera menu: hit menu button>audio input>trim CH-1;CH-2
>audio input>monitor CH>select
Remember, you have to be in manual mode on each input channel for the menu controls to be active.
You also cannot trim the internal mic(s) L&R, only mics connected externally (at least on the EX1R).