I was not able to install my purchased Rex4 Texture Direct + Softclouds downloaded zip file.
The solution is really simple: don't use Windows Explorer to extract the zip file. Instead use 7Zip or maybe Winzip. 7Zip solved this issue for me! It's because Windows Explorer didn't completely extract all files nicely (the 5th and 6th file were corrupted as a result - I tried this extraction many times, even a new download didn't solve the issue).
Please include this tip for other people downloading and installing this zip file as a separate installation.txt in the download section of SimMarket in the download section of your account. It was driving me nuts, but the solution is actually so simple. I hope other people will not run into this issue again.
Bye, bye, Richard Z.