When you click on the download link, the browser will open a new window and the download will start. After it is complete, save file to the local disk.
On your local computer, create a folder, where you would like to extract files from the downloaded .zip archive. Then, unzip files from the archive to this folder.
You may see warning messages from Windows Firewall and/or from an antivirus software that runs on your computer. The warnings will say that the file you are about to launch is suspicious. If you see these warnings, ignore them by choosing the option that runs opt_dev.exe anyway. This file contains no viruses!
Click on "More info" to see the "Run Anyway" option
Choose "run anyway" option
In case of successful launch, you should see this window on your screen:
If you see this application widow, you can start playing with OPT!