Version 1 says nothing on the menu about version, but version 2 has a version listed on the menu. If you purchased the game recently then it is most likely version 2. However if you purchased it from a download site like Direct2Drive or Steam you have version 3 and there is noway to mod version 3.
Go to the download section and get the downgrade patch. If you are running Vista you need to install your game on the desktop to modify it. XP is fine where you put it. Install the patch in the same directory as your gta_sa.exe and run the patch.
Now from my point of view if you are having this much trouble just doing the downgrade... why on Earth do you want to mod?
The downgrade link does not work when I click download, it brings me to an error page. Is there a link to patch?