How do I add an already built and textured set in unity?

I'm doing a project for school where I need to have a set for a video game, so I created a set with just the basics. I then zipped it and sent it to a classmate for them to texture and tweak. They did all that and then sent it back to me so I could add it back in to unity and add props. When I try to bring the file into unity, it brings up an error that says "Invalid Project Path". I'm not terribly familiar with unity and I would love some help with this!