If you are getting this error, then that means you have to decompress the package before opening it with UE Explorer.
You can do this using the tool from Gildors website called Unreal Package Decompresser: http://www.gildor.org/downloads
Enjoy!
You can do this using the tool from Gildors website called Unreal Package Decompresser: http://www.gildor.org/downloads
Enjoy!