In the welcome screen, show a hint/tips label explaining that you can drag & drop files and folders onto the filechooser buttons and comboboxes
Probably one of the best hidden secrets of Meld is that you don't have to use the cumbersome GTK FileChooser to pick what you want to compare... you can drag & drop directly from nautilus onto the two (or three) files (or folders) comparison buttons in the home screen.
Since there is plenty of space available, I think it would be a nice touch to inform users about the existence of that UI affordance, by showing a GtkLabel that says something like:
⭐Tip: You can drag and drop the %type from another application (such as a file manager) to the %type chooser buttons above to set your points of comparison.
Where %type = "files" or "folders" (depending on what mode the user is in). If you keep it simple without taking account the 2 vs 3 point of comparison variants, you could probably also get away with just having two translatable strings for the two scenarios instead of using variables in those strings.