Here's the link for the Outback Roadies posting thread, for future reference.
V1-Australia10K-OutbackRoadies.zip is the zipped World. It is a large file and zipped in this way to reduce the download size.
The hosting company doesn't like file sizes greater than 100mb either, and that's another reason for zipping it.
Step 1. Unzip V1-Australia10K-OutbackRoadies.zip into the Mods folder.
Step 2. Ensure the folder hierarchy is:
V1-Australia10K-OutbackRoadies as the top folder with a ModInfo.xml file inside.
If the hierarchy is V1-Australia10K-OutbackRoadies / V1-Australia10K-OutbackRoadies / ModInfo.xml is means it has nested another V1-Australia10K-OutbackRoadies.
If this happens, the World will not be visible in the World selection choices. Some zip programmes double nest files in this way.
Extract to
V1.0-OutbackRoadies\Mods\
and not
V1.0-OutbackRoadies\Mods\V1-Australia10K-OutbackRoadies.