Green Box: "Could not load file"

[Caltereth]

Hi there,

I'm pretty new to the community here, but a friend of me told me that this site had a lot of models and skins that I could download.
I've downloaded several models now, but I can't get them to work on World Editor. I'm doing exactly as the tutorial for importing models is telling me to do, but when I'm spawning the unit, I only get a green box and an error message "Error: Could not load file".

I really hope there's a way to fix this problem, cause I'd love to download some models together with some doodads for my current projects.