To install it, select "Install TrainCrew Application" from the JMRI Debug menu. Note that you'll need an active internet connection when you do this. But you won't need one to run TrainCrew after that.
Windows users will probably need to run JMRI with
Administrative permissions
(eg. right click on PanelPro desktop link,
Run as Administrator) to install.
it is not recommended to run JMRI as Administrator
all of the time, just to install TrainCrew.
TrainCrew code is available on GitHub here.