Hi sailfish and welcome to the forum.

What you're seeing means that either a 2D panel wasn't included in the package (rare but sometimes happens), or the panel folder is the wrong place.
Check the contents of the folder of the particular aircraft you're having a problem with and see if there's a panel folder inside it - that's where it should be.
If not, I'd advise going back to the original zip file you downloaded for the aircraft and unzipping it again. See if there's a panel folder there and pay particular attention to the readme document that should accompany the installation files. It should explain how to correctly install it there.