I opened a files I was working on and I wanted to add another .blend files to it. So I went to append the other file but when I clicked on the file I wanted to add and hit append, but nothing happened. Every time I tried to append it nothing would happen.
So normally the blender file manager would open if you hit append. You would open a .blend file and then you have the choice of a category e.g. object or material etc. Then you choose the object(s) (or material etc.).