You did not install the extension. The extension is installed when you install Pega Robotics. There is a radio button to select to install it. You will need to re-install Pega Robotics and select the radio button to install it for Chrome.
If you go to the installation folder for openspan, do you see a folder for 'Extensions'? The install files should be in there and you can drag the .crx file onto the chrome extensions window and it should manually install it.