first install this one: http://realenvironmentxtreme.com/forum/index.php?topic=4740.0
just run the setup and install it.
The second install is this one: http://realenvironmentxtreme.com/forum/index.php?topic=5091.0
Again just run the setup and install it.
The thrid service pack (SP3): can be downloaded here, http://realenvironmentxtreme.com/forum/index.php?topic=9542.0
Which one I dont know but I think you already have SP3 so you can install REX Overdrive. Do you have errors during that install, you can do all service packs again and then look for the good SP3. This install is not the same, you have to copy and paste the setup in your REX main directory and then run it from there as administrator. Tell us if it works.
TimFlightX