I read in quite a few posts that the values in the media drop-down list are user-defined, and can be changed/set via the preferences menu.
What I can't work out is how.

When I open the preferences menu all I get is a box that allows me to set my defaults. I can't see any way of being able to add a value to the list/s.
I would very much like to add a "digital" value to the media list, as around 70% of my movies are now not on disc.
Could somebody please explain the process to me in words that my poor aging brain can understand?