Quick question and probably very basic one that I don't know yet, please help

There is always a level file in the mode that when you take a look at the reference view it reference to some assets, but I only accidently made it work once, and I don’t know what I did was right, the rest of my mod I can never see the reference changes doesn’t matter how many items i put in the folder and change them. how to link them to the level file?