Trying to add Environment to project, assets do NOT show in content browser

I have seemingly added the assets to my project, but they do not show up in the content browser for me to use. The files are there on the explorer, but again, do not show up in the content manager. I have attempted many trouble shooting options. I am using UE 4.8.3. The environment I purchased is “Semi-Arid Desert Environment”.

Copying the files directly into the explorer does NOT work. I can NOT drag the files into the content browser, I get the “failed import, extension not recognized” error. I have tried 4.7 and 4.8; problem still persists. As I also mentioned, I tried using the “add to project” function. The files are successfully added, but are not showing in content browser. I deleted the “CachedAssetRegistry.bin” to recheck the assets, but that also failed.

Obviously, I am missing something obvious. I just need a push in the right direction.

Quite an annoying issue. I bought a product I can not use. Does anyone have any ideas?