Adding screen flash (Hitfilm 3 Express)
You need to switch to the compositing timeline in HitFilm so that you can animate your effect (using keyframes). There are 2 timelines in HitFilm - one for editing your footage and another for layered animation work.

It's worth watching the tutorials on the HitFilm YouTube channel here > https://www.youtube.com/user/FXhomeHitFilm

This specific tutorial explains the difference between the two timelines > https://youtu.be/bawBXIhMxsM

This video explains animating with keyframes > https://youtu.be/jEPm2FPMZI8

I hope that helps! :D