!!! It's best practice to add the folder to your project while it is NOT open, this seems to avoid random validation issues, then when you do open the project I recommend right clicking on the "ApolloRockMaster" folder and then selecting "Resave all" !!!
Replace each rock's texture data with the corresponding one included in the download, this is easy if you just use the name of the texture data already in place as reference (I've named all the texture data assets included in the zip the same way they're named in UEFN).