Just as in every windows program. Copy the highlighted line or lines (without using the context menu/mouse) en paste it somewhere for example in the top source box. The box just below the top source box is the only place in the program where such "standard" windows behaviour/shortcut doesn't work. (Only select all, ctrl-a is implemented, why not ctrl-c?).
Your right about the paste option. My mistake. Sorry.
I was a bit confused. The context copy menu item copies a text item. The context paste item takes file items.
BTW the paste shortcut key ctrl-v doesn't work here either.