Inside the game directory, Extract art.rcf and once complete delete art.rcf. This creates files in a folder called art.rf. Also, make sure to backup the original art.rcf file.

Now your mod is ready to be installed.

Just copy the contents of power pack into the prototype directory, you can just select "Browse Local Files" in steam and that's the place you put the files in.

It will ask you to replace the files in which case if you have a backup of the original art.rcf say yes. This way if the mod doesn't work you can always revert to your backup save.

If you don't have the extractor I will provide a link to go download it.